tl;dr I parodied my own book, download the new version here. People often complain to me about how olympiad geometry is just about knowing a bunch of configurations or theorems. But it recently occurred to me that when you actually … Continue reading

# Tag Archives: geometry

# New oly handout: Constructing Diagrams

I’ve added a new Euclidean geometry handout, Constructing Diagrams, to my webpage.

Some of the stuff covered in this handout:

- Advice for constructing the triangle centers (hint: circumcenter goes first)
- An example of how to rearrange the conditions of a problem and draw a diagram out-of-order
- Some mechanical suggestions such as dealing with phantom points
- Some examples of computer-generated figures

Enjoy.

# Revisiting arc midpoints in complex numbers

## 1. Synopsis

One of the major headaches of using complex numbers in olympiad geometry problems is dealing with square roots. In particular, it is nontrivial to express the incenter of a triangle inscribed in the unit circle in terms of its vertices.

The following lemma is the standard way to set up the arc midpoints of a triangle. It appears for example as part (a) of Lemma 6.23.

**Theorem 1** **(Arc midpoint setup for a triangle)**

Let be a triangle with circumcircle and let , , denote the arc midpoints of opposite , opposite , opposite .

Suppose we view as the unit circle in the complex plane. Then *there exist* complex numbers , , such that , , , and

Theorem 1 is often used in combination with the following lemma, which lets one assign the incenter the coordinates in the above notation.

**Lemma 2** **(The incenter is the orthocenter of opposite arc midpoints)**

Let be a triangle with circumcircle and let , , denote the arc midpoints of opposite , opposite , opposite . Then the incenter of coincides with the orthocenter of .

Unfortunately, the proof of Theorem 1 in my textbook is wrong, and I cannot find a proof online (though I hear that *Lemmas in Olympiad Geometry* has a proof). So in this post I will give a correct proof of Theorem 1, which will hopefully also explain the mysterious introduction of the minus signs in the theorem statement. In addition I will give a version of the theorem valid for quadrilaterals.

## 2. A Word of Warning

I should at once warn the reader that Theorem 1 is an *existence result*, and thus must be applied carefully.

To see why this matters, consider the following problem, which appeared as problem 1 of the 2016 JMO.

**Example 3** **(JMO 2016, by Zuming feng)**

The isosceles triangle , with , is inscribed in the circle . Let be a variable point on the arc that does not contain , and let and denote the incenters of triangles and , respectively. Prove that as varies, the circumcircle of triangle passes through a fixed point.

By experimenting with the diagram, it is not hard to guess that the correct fixed point is the midpoint of arc , as seen in the figure below. One might be tempted to write , , , and assert the two incenters are and , and that the fixed point is .

This is a mistake! If one applies Theorem 1 twice, then the choices of “square roots” of the common vertices and may not be compatible. In fact, they *cannot* be compatible, because the arc midpoint of opposite is different from the arc midpoint of opposite .

In fact, I claim this is not a minor issue that one can work around. This is because the claim that the circumcircle of passes through the midpoint of arc is false if lies on the arc on the same side as ! In that case it actually passes through instead. Thus the truth of the problem really depends on the fact that the quadrilateral is *convex*, and any attempt with complex numbers must take this into account to have a chance of working.

## 3. Proof of the theorem for triangles

Fix now, so we require , , . There are choices of square roots , , we can take (differing by a sign); we wish to show one of them works.

We pick an arbitrary choice for first. Then, of the two choices of , we pick the one such that . Similarly, for the two choices of , we pick the one such that . Our goal is to show that under these conditions, we have again.

The main trick is to now consider the arc midpoint , which we denote by . It is easy to see that:

**Lemma 4** **(The isosceles trapezoid trick)**

We have (both are perpendicular to the bisector). Thus is an isosceles trapezoid, and so .

Thus, we have

Thus

as desired.

From this we can see why the minus signs are necessary.

## 4. A version for quadrilaterals

We now return to the setting of a convex quadrilateral that we encountered in Example 3. Suppose we preserve the variables , , that we were given from Theorem 1, but now add a fourth complex number with . How are the new arc midpoints determined? The following theorem answers this question.

**Theorem 6** **( setup)**

Let be a convex quadrilateral inscribed in the unit circle of the complex plane. Then we can choose complex numbers , , , such that , , , and:

- The opposite arc midpoints , , of triangle are given by , , , as before.
- The midpoint of arc not including or is given by .
- The midpoint of arc not including or is given by .
- The midpoint of arc is and the midpoint of arc is .

This setup is summarized in the following figure.

Note that unlike Theorem 1, the four arcs cut out by the sides of do not all have the same sign (I chose to have coordinates ). This asymmetry is inevitable (see if you can understand why from the proof below).

*Proof:* We select , , with Theorem 1. Now, pick a choice of such that is the arc midpoint of not containing and . Then the arc midpoint of not containing or is given by

On the other hand, the calculation of for the midpoint of follows by applying Lemma 4 again. (applied to triangle ). The midpoint of is computed similarly.

In other problems, the four vertices of the quadrilateral may play more symmetric roles and in that case it may be desirable to pick a setup in which the four vertices are labeled in order. By relabeling the letters in Theorem 6 one can prove the following alternate formulation.

**Corollary 7**

Let be a convex quadrilateral inscribed in the unit circle of the complex plane. Then we can choose complex numbers , , , such that , , , and:

- The midpoints of , , , cut out by the sides of are , , , .
- The midpoints of and are and .
- The midpoints of and are and .

To test the newfound theorem, here is a cute easy application.

**Example 8** **(Japanese theorem for cyclic quadrilaterals)**

In a cyclic quadrilateral , the incenters of , , , are the vertices of a rectangle.

# Some Advice for Olympiad Geometry

I know some friends who are fantastic at synthetic geometry. I can give them any problem and they’ll come up with an incredibly impressive synthetic solution. I also have some friends who are very bad at synthetic geometry, but have such good fortitude at computations that they can get away with using Cartesian coordinates for everything.

I don’t consider myself either of these types; I don’t have much ingenuity when it comes to my solutions, and I’m actually quite clumsy when it comes to long calculations. But nonetheless I have a high success rate with olympiad geometry problems. Not only that, but my solutions are often very *algorithmic*, in the sense that any well-trained student should be able to come up with this solution.

In this article I try to describe how I come up which such solutions.

## 1. The Three Reductions

Very roughly, there are three different ways I try to make progress on a geometry problem.

- (I) The standard
**synthetic**techniques; angle chasing, cyclic quadrilaterals, homothety, radical axis / power of a point, etc. My own personal arsenal contains some weapons not known to many contestants as well, most notably inversion, harmonic bundles and quadrilaterals, and spiral similarity / Miquel points.For this part, it’s highly advantageous to be well-versed with “standard” configurations and tricks. To give an extreme example: to solve Iran TST 2009, Problem 9 one essentially needs only recognize two configurations: a lemma about the midpoint of an altitude (2002 G7) and another lemma about the line (USAJMO 2014/6). Not knowing either of these makes it more difficult to solve the problem synthetically in the time limit. As a reference, Yufei Zhao’s lemmas handout contains a fairly comprehensive list of these configurations.Easier problems don’t require as much in this way of configuration recognition.

- (II) Standard
**computational**techniques (aka bashing). Personally, I prefer complex numbers and barycentric coordinates but I know other students who will use Cartesian coordinates and trigonometry to great success. The advantage of such methods is that they are**straightforward**and**reliable**, albeit tedious and time-consuming. It is mostly a matter of experience to understand whether a calculation can be carried out within the time limit — I can basically tell just by looking at a setup whether it can be solved in this time. - (III) Most surprisingly: simply
**finding crucial claims**. Especially for harder problems like IMO 3/6 much of the time the key to solving a problem is making some key observation. Said another way: a difficult IMO 3/6 problem which asks you to prove might have a solution which goes like,Each of the individual implications might be no harder than an IMO 1/4 but the difficulty rests in finding what to prove. The most reliable way to do such things is to draw

**large, in-scale diagrams**. If you are good at recognizing cyclic quadrilaterals, collinear points, etc. then the correct claims will naturally suggest themselves; conversely, good diagrams will prevent you from wasting time trying to prove things that aren’t true (effectively letting you test your claims “experimentally” before trying to prove them).

Type (III) deserves some comment here. There is more to making progress on a problem than simply trying things you think will solve the problem: there is some “scouting” involved that you will need to do for any difficult problems. As a terrible analogy, in StarCraft you have to scout an experienced opponent to understand what they’re doing before you try to attack them. The situation with IMO 3/6 is no different: you have to have some understanding of the problem before you stand a chance of being able to solve it.

Easy problems can often succumb to just one class of attacks, but the interesting and difficult problems can require two or all three classes in order to solve. How much you use each type of strategy is in my opinion a matter of personal taste — some people don’t use (II) at all and rely on (I) to prove everything, and even vice versa! I like to think I balance (I) and (II) evenly. But (III) is indispensable, and in any case I think part of the reason I have been so successful with geometry problems is precisely that I can draw on all three strategies in tandem, rather than being limited to one or two.

In fact, a good rule of thumb that I use for judging the difficulty of a problem is how many of the above methods I had to use: the th problem on an IMO paper should require me to resort to about of these strategies.

## 2. Concrete Examples

I’ll now give some concrete examples of the things I said above. Warning: spoilers follow, and hyperlinks lead to my solutions on Art of Problem Solving. You are encouraged to try the problems yourself before reading the comments.

Example[EGMO 2012/1] Let be a triangle with circumcenter . The points , , lie in the interiors of the sides , , respectively, such that is perpendicular to and is perpendicular to . Let be the circumcenter of triangle . Prove that the lines and are perpendicular.

This is a pretty typical entry-level geometry problem. Do some angle chasing (I) to find one cyclic quad (III), and then follow through to solve the problem (I). If you are good enough, you don’t even need to find the cyclic quad in advance; just play around with the angles until you notice it.

Example[IMO 2014, Problem 4] Let and be on segment of an acute triangle such that and . Let and be the points on and , respectively, such that is the midpoint of and is the midpoint of . Prove that the intersection of and is on the circumference of triangle .

You can solve this problem by barycentric coordinates (II) instantly (textbook example). Also similar triangles (I) solves the problem pretty quickly as well. Again, this problem is “easy” in the sense that one can directly approach it with either (I) or (II), not needing (III) at all.

Example[USAMO 2015/2] Quadrilateral is inscribed in circle with and . Let be a variable point on segment . Line meets again at (other than ). Point lies on arc of such that is perpendicular to . Let denote the midpoint of chord . As varies on segment , show that moves along a circle.

This was not supposed to be a very difficult problem, but it seems to have nearly swept the JMO group. Essentially, the key to this problem is to notice that the center of the desired circle is in fact the midpoint of (with the center of the circle). This is a huge example of (III) — after this observation, one can solve the problem very quickly using complex numbers (II). It is much harder (though not impossible) to solve the problem without knowing the desired center.

Example[USAMO 2014/5] Let be a triangle with orthocenter and let be the second intersection of the circumcircle of triangle with the internal bisector of the angle . Let be the circumcenter of triangle and the orthocenter of triangle . Prove that the length of segment is equal to the circumradius of triangle .

Personally I think the most straightforward solution is to use (I) to eliminate the orthocenter condition, and then finish with complex numbers (II). Normally, you won’t see a medium-level problem that dies immediately to (II), and the only reason a problem like this could end up as a problem 5 is that there is a tiny bit of (I) that needs to happen before the complex numbers becomes feasible.

Example[IMO 2014/3] Convex quadrilateral has . Point is the foot of the perpendicular from to . Points and lie on sides and , respectively, such that lies inside triangle and

Prove that line is tangent to the circumcircle of triangle .

Like most IMO 3/6’s I had to resort to using all three methods in order to solve this problem. The first important step was finding out what to do with the angle condition. It turns out that in fact, it’s equivalent to the circumcenter of triangle lying on side of the triangle (III); proving this is then a matter of angle chasing (I). Afterwards, one has to recognize a tricky usage of the angle bisector theorem (I) to reduce it to something that can be computed with trigonometry (II). This leads to a direct solution that, while not elegant, also requires much less ingenuity then most of the solutions found by friends I know.

I really want to stress that being proficient in all three strategies is key to getting “straightforward” solutions like this to IMO 3/6 caliber problems. If you miss any of these components, you are not going to solve the problem.

Example[IMO 2011/6] Let be an acute triangle with circumcircle . Let be a tangent line to , and let , , be the lines obtained by reflecting in the lines , , and , respectively. Show that the circumcircle of the triangle determined by the lines , , and is tangent to the circle .

The ultimate example of these three principles. Using a trick that showed up on APMO 2014/5 and RMM 2013/3, one constructs the tangency point and connects the points , , , as I explain in this post, yielding points , , . After that, a very careful examination of the diagram (possibly several diagrams) leads to a conjecture that , et cetera. This is the key observation (III), and leads to highly direct solution via (II). But the point of this problem is that you need to *have the guts* to construct those auxiliary points and then boldly claim they are the desired “squared” points.

## 3. Comparison with Other Subjects

The approaches I’ve described highlight some of the features of olympiad geometry which distinguish it from other subjects.

- Unlike other olympiad subjects, you can actually obtain a big advantage by just knowing lots of theory. Experienced contestants simply “recognize” a large body of common configurations that those without access to training materials have never seen before. Similarly, there are a lot of fancy techniques that can make a big difference. This is much less true of other subjects (for example combinatorics is the opposite extreme).
- There’s less variance in the subject: lots of Euclidean geometry problems feel the same, and all of them use the same body of techniques. It reminds me of chess: it’s very “narrow” in the sense that at the end of the day, there are only so many possible moves. (Olympiad inequalities also has this kind of behavior.) Again combinatorics is the opposite of this.
- You have a reliable backup in case you can’t find the official solution: bash. Moreover, in general there are often many different ways to solve a problem; not true of other subjects.
- If you want to make some “critical claim” you can quickly test it empirically (by drawing a good diagram).

# Writing Olympiad Geometry Problems

*You can use a wide range of wild, cultivated or supermarket greens in this recipe. Consider nettles, beet tops, turnip tops, spinach, or watercress in place of chard. The combination is also up to you so choose the ones you like most.*

— Y. Ottolenghi. Plenty More

In this post I’ll describe how I come up with geometry proposals for olympiad-style contests. In particular, I’ll go into detail about how I created the following two problems, which were the first olympiad problems which I got onto a contest. Note that I don’t claim this is the only way to write such problems, it just happens to be the approach I use, and has consistently gotten me reasonably good results.

**[USA December TST for 56th IMO]** Let be a triangle with incenter whose incircle is tangent to , , at , , , respectively. Denote by the midpoint of and let be a point in the interior of so that and . Let be a point on the incircle such that . Prove that either or .

**[Taiwan TST Quiz for 56th IMO]** In scalene triangle with incenter , the incircle is tangent to sides and at points and . The tangents to the circumcircle of at and meet at . Lines and intersect at . Prove that the circle with diameter is orthogonal to the nine-point circle of .

## 1. General Procedure

Here are the main ingredients you’ll need.

- The ability to consistently solve medium to hard olympiad geometry problems. The intuition you have from being a contestant proves valuable when you go about looking for things.
- In particular, a good eye: in an accurate diagram, you should be able to notice if three points look collinear or if four points are concyclic, and so on. Fortunately, this is something you’ll hopefully have just from having done enough olympiad problems.
- Geogebra, or some other software that will let you quickly draw and edit diagrams.

With that in mind, here’s the gist of what you do.

- Start with a configuration of your choice; something that has a bit of nontrivial structure in it, and add something more to it. For example, you might draw a triangle with its incircle and then add in the excircle tangency point, and the circle centered at passing through both points (taking advantage of the fact that the two tangency points are equidistant from and ).
- Start playing around, adding in points and so on to see if anything interesting happens. You might be guided by some actual geometry constructions: for example, if you know that the starting configuration has a harmonic bundle in it, you might project this bundle to obtain the new points to play with.
- Keep going with this until you find something unexpected: three points are collinear, four points are cyclic, or so on. Perturb the diagram to make sure your conjecture looks like it’s true in all cases.
- Figure out why this coincidence happened. This will probably add more points to you figure, since you often need to construct more auxiliary points to prove the conjecture that you have found.
- Repeat the previous two steps to your satisfaction.
- Once you are happy with what you have, you have a nontrivial statement and probably several things that are equivalent to it. Pick the one that is most elegant (or hardest), and erase auxiliary points you added that are not needed for the problem statement.
- Look for other ways to reduce the number of points even further, by finding other equivalent formulations that have fewer points.

Or shorter yet: build up, then tear down.

None of this makes sense written this abstractly, so now let me walk you through the two problems I wrote.

## 2. The December TST Problem

In this narrative, the point names might be a little strange at first, because (to make the story follow-able) I used the point names that ended up in the final problem, rather than ones I initially gave. Please bear with me!

I began by drawing a triangle (always a good start\dots) and its incircle, tangent to side at . Then, I added in the excircle touch point , and drew in the circle with diameter , which was centered at the midpoint . This was a coy way of using the fact that ; I wanted to see whether it would give me anything interesting.

So, I now had the following picture.

Now I had two circles intersecting at a single point , so I added in , the second intersection. But really, this point can be thought of another way. If we let be the diameter of the incircle, then as is the other diameter, is actually just the foot of the altitude from to line .

But recall that , , are collinear! (Again, this is why it’s helpful to be familiar with “standard” contest configurations; you see these kind of things immediately.) So in fact lies on line .

This was pretty cool, though not yet interesting enough to be a contest problem. So I looked for most things that might be true.

I don’t remember what I tried next; it didn’t do anything interesting. But I do remember the thing I tried after that: I drew in the angle bisector, line . And then, I noticed a big coincidence: the first intersection of with the circle with diameter seemed to lie on line ! I was initially confused by this; it didn’t seem like it could possibly be true due to symmetry reasons. But in my diagram, it was indeed correct. A moment later, I realized the reason why this was plausible: in fact, the second intersection of line with the circle was on line .

Now, I could not see quickly at all why this was true. So I started trying to prove it, but initially failed: however, I managed to show (via angle chasing) that

So, at least I had an interesting equivalent statement.

After another half hour of trying to prove my conjecture, I finally realized what was happening. The point was the one attached to a particular lemma: the -bisector, -midline, and touch-chord are concurrent, and from this just follows by some similar triangles. So, drawing in the point (the midpoint of ), I had the full configuration which gave the answer to my conjecture.

Finally, I had to clean up the mess that I had made. How could I do this? Well, the points , could be eliminated easily enough. And we could re-define to be a point on the incircle such that . This actually eliminated the green circle and point altogether, provided we defined by just saying that it was on the angle bisector, and that . (So while the circle was still implicit in the condition , it was no longer explicitly part of the problem.)

Finally, we could even remove the line through , and ; we ask the contestant to prove .

And that was it!

## 3. The Taiwan TST Problem

In fact, the starting point of this problem was the same lemma which provided the key to the previous solution: the circle with diameter intersects the and bisectors on the touch chord. Thus, we had the following diagram.

The main idea I had was to look at the points , , in conjunction with each other. Specifically, this was the orthic triangle of , a situation which I had remembered from working on Iran TST 2009, Problem 9. So, I decided to see what would happen if I drew in the nine-point circle of . Naturally, this induces the midpoint of .

At this point, notice (or recall!) that line is concurrent with lines and .

So the nine-point circle of the problem is very tied down to the triangle . Now, since I was in the mood for something projective, I constructed the point , the intersection of lines and . In fact, what I was trying to do was take perspectivity through . From this we actually deduce that is a harmonic bundle.

Now, what could I do with this picture? I played around looking for some coincidences, but none immediately presented themselves. But I was enticed by the point , which was somehow related to the cyclic complete quadrilateral . So, I went ahead and constructed the pole of to the nine-point circle, letting it hit line at . This was aimed at “completing” the picture of a cyclic quadrilateral and the pole of an intersection of two sides. In particular, was harmonic too.

I spent a long time thinking about how I could make this into a problem. I unfortunately don’t remember exactly what things I tried, other than the fact that I was taking a lot of perspectivity. In particular, the “busiest” point in the picture is , so it makes sense to try and take perspectives through it. Especially enticing was the harmonic bundle

How could I use this to get a nice result?

Finally about half an hour I got the right idea. We could take this bundle and intersect it with the ray ! Now, letting be the midpoint , we find that three of the points in the harmonic bundle we obtain are , , and ; let be the fourth point, which is the intersection of line with . Then by hypothesis, we ought to have . But from this we know exactly what the point . Just look at the circumcircle of triangle : as this has diameter , we see that is the intersection of the tangents at and .

Consequently, we know that the point , defined very naturally in terms of the original picture, lies on the polar of to the nine-point circle. By simply asking the contestant to prove this, we thus eliminate all the points , , , , , , and completely from the picture, leaving only the nine-point circle. Finally, instead of directly asking the contestant to show that lies on the polar of , one can rephrase the problem as saying “the circle with diameter is orthogonal to the nine-point circle of ”, concealing all the work that went into the creation of the problem.

Fantastic.

# Constructing the Tangent and Cotangent Space

This one confused me for a long time, so I figured I should write this down before I forgot again.

Let be an abstract smooth manifold. We want to define the notion of a **tangent vector** to at a point . With that, we can define the **tangent space** , which will just be the (real) vector space of tangent vectors at .

Geometrically, we know what this *should* look like for our usual examples. For example, if is a circle embedded in , then the tangent vector at a point should just look like a vector running off tangent to the circle.

Similarly, given a sphere , the tangent space at a point along the sphere would look like plane tangent to at .

However, the point of an abstract manifold is that we want to see the manifold as an *intrinsic object*, in its own right, rather than as embedded in . This can be thought of as analogous to the way that we think of a group as an abstract object in its own right, even though Cayley’s Theorem tells us that any group is a subgroup of the permutation group. (This wasn’t always the case! During the 19th century, a group was literally defined as a subset of or of . In fact Sylow developed his theorems without the word “group” Only much later did the abstract definition of a group was given, an abstract set which was independent of any *embedding* into , and an object in its own right.) So, we would like our notion of a tangent vector to not refer to an ambient space, but only to intrinsic properties of the manifold in question.

So how do we capture the notion of the tangent to a manifold referring just to the manifold itself? Well, the smooth structure of the manifold lets us speak of smooth functions . In the embedded case, we can thus think of taking a *directional derivative* along (i.e. some partial derivative). To give a concrete example, suppose we have a smooth function and a point . By the structure of a manifold, near the point , looks like a function on some neighborhood of the origin in So we are allowed to take the partial derivative of with respect to any of the vectors in .

For a fixed this partial derivative is a linear map . It turns out this goes the other way: if you know what does to every smooth function, then you can figure out which vector it’s taking the partial derivative of. This is the trick we use in order to create the tangent space. Rather than trying to specify a vector directly (which we can’t do because we don’t have an ambient space), we instead look at *arbitrary* derivative-like functions, and associate them with a vector. More formally, we have the following.

**Definition 1**

A **derivation** at is a linear map (i.e. assigning a real number to every smooth ) satisfying the following Leibniz rule: for any , we have the equality

This is just a “product rule”. Then the tangent space is easy to define:

**Definition 2**

A **tangent vector** is just a derivation at , and the **tangent space** is simply the set of all these tangent vectors.

In fact, one can show that the product rule for is equivalent to the following three conditions:

- is linear, meaning .
- , where is the constant function on .
- whenever . Intuitively, this means that if a function vanishes to second order at , then its derivative along should be zero.

This suggests a third equivalent definition: suppose we define

to be the set of functions which vanish at (this is called the *maximal ideal* at ). In that case,

is the set of functions vanishing to second order at . Thus, a tangent vector is really just a linear map

In other words, the tangent space is actually the dual space of ; for this reason, the space is defined as the **cotangent space** (the dual of the tangent space). This definition is even more abstract than the one with derivations above, but it has the advantage (or so I’m told) that it can be transferred to other settings (like algebraic varieties).

*EDIT (Oct 5 2015)*: Reproducing this Reddit comment by tactics, The beauty of the definition given in this blog post is stuffed away into an easily-forgotten sentence. This definition:

- Does not rely on any kind of parametrization, and
- It is defined only in terms of the ring of “regular functions” defined on the space.

The former is nice for philosophical reasons. The latter is nice because we can pull in a lot of intuition about manifolds into the study of algebraic varieties, and similarly, we can inject a lot of ring theory into the study of manifolds.

With all these equivalent definitions, the last thing I should do is check that this definition of tangent space actually gives a vector space of dimension . To do this it suffices to show verify this for open subsets of , which will imply the result for general manifolds (which are locally open subsets of ). Using some real analysis, one can prove the following result:

**Theorem 3**

Suppose is open and . Then

In other words is the set of functions which vanish at and such that all first derivatives of vanish at zero.

Thus, it follows that there is an isomorphism

and so the cotangent space, hence tangent space, indeed has dimension .

# The Mixtilinear Incircle

*This blog post corresponds to my newest olympiad handout on mixtilinear incircles.*

My favorite circle associated to a triangle is the -mixtilinear incircle. While it rarely shows up on olympiads, it is one of the richest configurations I have seen, with many unexpected coincidences showing up, and I would be overjoyed if they become fashionable within the coming years.

Here’s the picture:

The points and are the contact points of the incircle and -excircle on the side . Points , , are the midpoints of the arcs.

As a challenge to my recent USAMO class (I taught at A* Summer Camp this year), I asked them to find as many “coincidences” in the picture as I could (just to illustrate the richness of the configuration). I invite you to do the same with the picture above.

The results of this exercise were somewhat surprising. Firstly, I found out that students without significant olympiad experience can’t “see” cyclic quadrilaterals in a picture. Through lots of training I’ve gained the ability to notice, with some accuracy, when four points in a diagram are concyclic. This has taken me a long way both in setting problems and solving them. (Aside: I wonder if it might be possible to train this skill by e.g. designing an “eyeballing” game with real olympiad problems. I would totally like to make this happen.)

The other two things that happened: one, I discovered one new property while preparing the handout, and two, a student found yet another property which I hadn’t known to be true before. In any case, I ended up covering the board in plenty of ink.

Here’s the list of properties I have.

- First, the classic: by Pascal’s Theorem on , we find that points , , are collinear; hence the contact chord of the -mixtilinear incircle passes through the incenter. The special case of this problem with appeared in IMO 1978.
- Then, by Pascal on , we discover that lines , , and are also concurrent.
- This also lets us establish (by angle chasing) that and are concyclic. In addition, lines and are tangents to these circumcircles at (again by angle chasing).
- An Iran 2002 problem asks to show that ray passes through the point diametrically opposite on the circumcircle. This is solved by noticing that is a symmedian of the triangle and (by the previous fact) that is a median. This is the key lemma in Taiwan TST 2014, Problem 3, which is one of my favorite problems (a nice result by Cosmin Pohoatza).
- Lines and are isogonal. This was essentially EGMO 2012, Problem 5, and the “morally correct” solution is to do an inversion at followed by a reflection along the -bisector (sometimes we call this a “ inversion”).
- As a consequence of this, one can also show that lines and are isogonal (with respect to ).
- One can also deduce from this that the circumcircle of passes through the intersection of and .

- Lines and meet on the mixtilinear incircle. (Homothety!)
- Moreover, line passes through the exsimilicenter of the incircle and circumcircle, by, say Monge d’Alembert. Said another way, the mentioned exsimilicenter is the isogonal conjugate of the Nagel point.

To put that all into one picture:

# Three Properties of Isogonal Conjugates

In this post I’ll cover three properties of isogonal conjugates which were only recently made known to me. These properties are generalization of some well-known lemmas, such as the incenter/excenter lemma and the nine-point circle.

**1. Definitions **

Let be a triangle with incenter , and let be any point in the interior of . Then we obtain three lines , , . Then the reflections of these lines across lines , , always concur at a point which is called the **isogonal conjugate** of . (The proof of this concurrence follows from readily from Trig Ceva.) When lies inside , then is the point for which and so on.

The isogonal conjugate of is sometimes denoted . Note that .

**Examples** of pairs of isogonal conjugates include the following.

- The incenter is its own isogonal conjugate. Similarly, each excenter is also its own isogonal conjugate.
- The isogonal conjugate of the circumcenter is the orthocenter.
- The isogonal conjugate of the centroid is the symmedian point.
- The isogonal conjugate of the Nagel point is the point of concurrence of , , , where is the contact point of the –mixtilinear incircle. The proof of this result was essentially given as Problem 5 of the European Girl’s Math Olympiad.

**2. Inverses and Circumcircles **

You may already be aware of the famous result (which I always affectionately call “Fact 5”) that the circumcenter of is the midpoint of arc of the circumcircle of . Indeed, so is the circumcenter of triangle , where is the -excenter.

In fact, it turns out that we can generalize this result for arbitrary isogonal conjugates as follows.

Theorem 1Let and be isogonal conjugates. Then the circumcenters of and are inverses with respect to the circumcircle of .

*Proof:* This is just angle chasing. Let and be the desired circumcenters. It’s clear that both and lie on the perpendicular bisector of . Angle chasing allows us to compute that

Similarly, . But the reader can check that . Using this we can show that , so , as needed.

When we take and to be (or ), we recover the Fact 5 we mentioned above. When we take to be the orthocenter and to be the circumcenter, we find that the circumcenter of is the inverse of the circumcenter of . But the inverse of the circumcenter of is the reflection of over . Thus we derive that and have circumcircles which are just reflections over .

**3. Pedal Circles **

You may already be aware of the nine-point circle, which passes through the midpoints and feet of the altitudes of . In fact, we can obtain such a circle for any pair of isogonal conjugates.

Theorem 2Let and be isogonal conjugates in the interior of . The pedal triangles of and share a circumcircle. Moreover, the center of this circle is the midpoint of .

Upon taking and we recover the nine-point circle. Of course, the incircle is the special case !

*Proof:* Let and be the pedal triangles. We leave the reader to check that

Consequently, the points , , , are concyclic. The circumcenter of these four points is the intersection of the perpendicular bisectors of segments and , which is precisely . Thus

Similarly work with the other vertices shows that is indeed the desired circumcenter.

There is a second way to phrase this theorem by taking a homothety at .

CorollaryIf the point is reflected about the sides , , and , then the resulting triangle has circumcenter .

**4. Ellipses **

We can actually derive the following remarkable result from the above theorem.

Theorem 3An ellipse is inscribed in triangle . Then the foci and are isogonal conjugates.

Of course, the incircle is just the special case when the ellipse is a circle.

*Proof:* We will deduce this from the corollary. Let the ellipse be tangent at points , , . Moreover, let the reflection of about the sides of be points , , . By definition, there is a common sum with

Because of the tangency condition, the points , , are collinear. But now

and we deduce

So is the circumcenter of . Hence is the isogonal conjugate of .

The converse of this theorem is also true; given isogonal conjugates and inside we can construct a suitable ellipse. Moreover, it’s worth noting that the lines , , are also concurrent; one proof is to take a projective transformation which sends the ellipse to a circle.

Using this theorem, we can give a “morally correct” solution to the following problem, which is IMO Shortlist 2000, Problem G3.

ProblemLet be the circumcenter and the orthocenter of an acute triangle . Show that there exist points , , and on sides , , and respectively such that

and the lines , , and are concurrent.

*Proof:* Because and are isogonal conjugates we can construct an ellipse tangent to the sides at , , from which both conditions follow.

**5. Pascal’s Theorem **

For more on isogonal conjugates, see e.g. Darij Grinberg. I’ll just leave off with one more nice application of isogonal conjugates, communicated to me by M Kural last August.

Theorem 4 (Pascal)Let by a cyclic hexagon, as shown. Suppose , and . Then points , , are collinear.

*Proof:* Notice that , though the triangles have opposite orientations. Because , and so on, the points and correspond to isogonal conjugates. Hence , which gives the collinearity.

*Thanks to R Alweiss and heron1618 for pointing out a few typos, and *Daniel Paleka for noticing a careless application of Brianchon’s theorem.

# Writing Olympiad Geometry

I always wondered whether I could generate olympiad geometry problems by simply drawing lines and circles at random until three lines looked concurrent, four points looked concyclic, et cetera. From extensive experience you certainly get the feeling that this ought to be the case — there are tons and tons of problems out there but most of them have relatively simple statements, not involving more than a handful of points. Often I think, “I bet I could have stumbled upon this result just by drawing things at random”.

So one night, I decided to join the tangency point of A-mixtilinear circle with the orthocenter of a triangle ABC. You can guess about how well that went. Nothing came up after two hours of messing around randomly.

Surprisingly, though, I found almost by accident that the following modification has had significant succes:

- First, start with a known configuration: for example, the the incircle and A-excircle touch BC at D, E such that BD = CE.
- Do some random manipulations to obtain an equivalent result. For this step, I like to use harmonic bundles, but I suspect other things work too. At the end of this step you should have something you might call a “proxy problem” — while you could take this and propose it to an olympiad, it’s probably somewhat contrived and uninteresting at the moment. (Your mileage may vary! I did get a nice result out of this once.)
- Starting with the proxy problem, start adding in new lines and circles and points until you find a conjecture.

In other words, it seems like “randomly wander” doesn’t work so well, but “randomly wander starting from somewhere that already has structure” works great. I’ve tried this four times and each time has gotten me a new problem.

I have to wonder what makes this work. I think it has something to do with the fact that olympiad geometry has a different structure to it than the other olympiad subjects. For example, given an arbitrary geometry problem and two contestants, it’s much more likely that they will come up with different solutions than in any other subject. This seems to suggest that, if one takes a true result and examines the diagram, there is likely other structure present within.

That’s my best guess for why this works, anyways. Obviously just heuristics, but as long as I keep getting more problems…

# Constructing Parallelograms

This is a reflection of a talk I gave today. Hopefully these reflections (a) help me give better talks, and (b) help out some others.

Today I was worked from 6PM-8PM with the Intermediate group at the Berkeley Math Circle, middle school students maybe one or two standard deviations above the average honors student. My talk today was “All you have to do is construct a parallelogram!”. Here is a link to the handout problems and their solutions. (Obviously I only went over a very proper subset of the problems during the lecture.)

### Background

Some background information: I had actually given an abridged version of the lecture to the honors geometry class at my Horner Junior High (discussing only 1,2,4,10). It had gone, as far as I could tell, very well. The HJH students audibly reacted as I completed the (short) solutions to their problems, meaning they not only understood the solutions but also could see them clearly enough to appreciate their elegance. A lot of students also thanked me after the lecture, and one of them asked for copies of my notes and told me I gave the best lectures. (Yay!)

The success of this lecture led me to adopt it for the Circle. The changes I made were:

- Add more problems (duh).
- Give more depth to the explanations, e.g. trying to explain motivations behind steps.
- Give students substantial time to try the problems themselves. Because IMO it spoils the fun for me to just keep giving solutions.

I plotted out in my head what direction the lecture would lead to. The most memorable part, I decided, would be problem #10 (by far my favorite on the set — try it!), and so I made a note to myself to end with that one. This turned out to be a good decision.

### Summary

I started out by asking the students to name some properties of parallelograms. The important ones came out — parallel sides, congruent sides, and bisection of diagonals, along with some others I didn’t think of. I got a lot of people trying to answer this one. So far so good.

I then asked what conditions listed above were sufficient to prove a parallelogram. Again I got the two I wanted: two sides both parallel and congruent, and the bisection of diagonals. I then explained that we were going to be using the bisection criteria a lot. I proceeded to present problem 1, asking the students for ideas, and was pleasantly surprised to end up with a solution that I hadn’t thought of. After congratulating the person who found the solution, I proceeded to present my solution, and tried to explain the idea of reflecting over midpoints to construct parallelograms.

I then let the students work for about 10 minutes on #2, #3, #4. Unfortunately, this is where the lecture started turning downhill. I walked around the room to see if there were any questions and to see what the students were coming up with. To my surprise, I found that a large portion of the students (more than half) were not doing the problems I asked them to do. I usually don’t mind if a few students aren’t interested, but the number of them really surprised me.

The second unpleasant surprise came when I began presenting the solution to #2. No one solved it in the 10 minutes, despite me having just told them to reflect points over midpoints (and there was only one possible reflection) — or if someone did, he/she didn’t saying anything. In retrospect I definitely should have written this on the board. Fortunately, I do coax the answer out of them by asking the right questions, but I now started to worry that the difficulty was too hard — ironic since I was initially worried about it being too easy.

The lack of solves on #2 means no one attempted #3 either (I also asked), so I am starting on a blank slate. Fortunately, this one turned out to be not as hard. The students quickly identify the good point and the steps towards the solution. #4 did not work as well, but this is mostly my fault because I made a mistake while presenting the solution and confused the students (and also kept mislabelling points). I did not catch it until the end. After the solution to #4 I am a bit nervous, as the students have been in general rather quiet and were not reacting at all to the solutions (unlike the Horner students). I briefly ask the students whether I am making sense, if I am going too fast/slow, etc. (Of course, I know from experience that this is about as useful as asking someone what he/she wants for Christmas, but I cannot help doing so anyways.)

At this point it is 6:55 or so, and we have our 10-minute break. Herein, one student asks me for help with word problems — I tell him to chat with me after break (he never did). The other asks me if I am v_Enhance on AoPS. I guess my reputation precedes me.

After the break we present monthly contest awards (guess who’s the grader?). I then give the students a substantial amount of time (20 mins or so) to work on #6, #7, #10, #11. The ones that did seem to be working seemed mostly interested in #10 (good!). However, as before, less than half the students were actually doing the problems. The others are chatting quietly or drawing in their notebooks. Again I say nothing, but at this point I am slightly discouraged.

At 7:30PM I regroup and begin discussing #6. This one went very badly. The students seemed confused in general, and mostly fail to answer my questions. It is at this point I inadvertently learn that a few students do not actually know the similarity theorem for triangles (SAS~, SSS~, AA). By the time I finish, 20 minutes have passed (leaving 10 left) and the students seem tired.

At this point I start with #10, which is met with some enthusiasm as quite a few students are eager to see a solution. I start by asking the students to randomly guess where to place the fourth vertex until they get it right. (There are only possible reflections, after all… :P). Nicely, the correct construction comes out on the fourth guess. :) I then proceed to explain the solution, which seems to be making much more sense. The timing works out great, and I deliver the punch line of the solution just as the lecture ends, to quite some applause. (You’ll have to see the solution to understand why!) Among the murmuring afterwards I hear lots of students saying that it was really good choice for ending. Quite a few students also thank me for the lecture afterwards (something that is actually quite rare), so it looks like these last ten minutes were more successful than anything else.

### Analysis

The thing that surprised me the most was the drawing in notebooks. I had assumed that the BMC students would be more motivated/interested than the kids at HJH. It turned out this was not the case. In retrospect, here are perhaps a few explanations why.

- At a public school, the type of lecture I gave is really rare and stands out. In contrast, the BMC students regularly get good lectures on good mathematics, so it was harder for me to stand out.
- I didn’t have the position of authority a classroom teacher has at BMC.
- The HJH honors geometry students are actually very strong, as these are precisely the students that skipped a year of math. Many, if not most, are also my students in math club.
- In that vein, many of the HJH students already know me well, from math club or otherwise.
- The students who would have been the top students in Intermediate were long promoted to the Advanced group.

Here are some other thoughts.

- I saw a lot of students staring at the diagrams I had provided on the page and not doing anything else. I wonder if next time I should omit diagrams from my geometry lectures, forcing the students to draw the diagrams themselves.
- USAMO problems seem too hard under any circumstances. Even when the phrase “this is not a very hard geometry problem” appears in the rubric, and with significant hints.
- I should have asked about the similarity. I kind of assumed everyone knew SSS~, SAS~, and AA. This was false.

A couple more positive ones.

- A couple times in the lecture I would ask the students for a Greek letter to name an angle. This actually drew a lot of attention and seemed to help with keeping the students awake; I should try this more in the future.
- The #10 at the end really saved my lecture. So I will be thinking some more about keeping an “ace in the hole” so my talks finish on a memorable note.

Eh, teaching is hard.