# 2016 USAJMO Problems/Problem 1

## Problem

The isosceles triangle $\triangle ABC$, with $AB=AC$, is inscribed in the circle $\omega$. Let $P$ be a variable point on the arc $\stackrel{\frown}{BC}$ that does not contain $A$, and let $I_B$ and $I_C$ denote the incenters of triangles $\triangle ABP$ and $\triangle ACP$, respectively.

Prove that as $P$ varies, the circumcircle of triangle $\triangle PI_BI_C$ passes through a fixed point.

## Solution

We will use complex numbers, with the circumcircle of $\triangle ABC$ as the unit circle. Let $$A=1, B=w^2,C=\frac{1}{w^2}, P=p^2,$$ such that $$I_B=w+wp-p, I_C=p-\frac{p}{w}+\frac{1}{w}.$$ We claim that the circumcircle of $\triangle PI_BI_C$ passes through $M=-1.$ This is true iff $$k=\frac{(I_B-M)(I_C-P)}{(I_B-P)(I_C-M)}=\frac{(w+wp-p+1)(p+\frac{1}{w}-\frac{p}{w}-p^2)}{(w+wp-p-p^2)(p+\frac{1}{w}-\frac{p}{w}+1)}=\frac{(wp+1)(1-p)}{(p+1)(w-p)}$$ is real. This is true iff $k=\overline{k}.$ We can compute $$\overline{k}=\frac{(1+\frac{1}{wp})(1-\frac{1}{p})}{(\frac{1}{p}+1)(\frac{1}{w}-\frac{1}{p})}=k,$$ so we are done. $\blacksquare$