2016 USAJMO Problems/Problem 1
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.
We claim that (midpoint of arc ) is the fixed point. We would like to show that , , , are cyclic.
We extend to intersect again at R. We extend to intersect again at S.
We invert around a circle centered at with radius (for convenience). (I will denote X' as the reflection of X for all the points) The problem then becomes: Prove , , and are collinear.
Now we look at triangle . We apply Menelaus (the version where all three points lie outside the triangle). It suffices to show that
By inversion, we know for any point and for any points and .
Plugging this into our Menelaus equation we obtain that it suffices to show We cancel out the like terms and rewrite. It suffices to show We know that is the diameter of because is isosceles and is the angle bisector. We also know so and are symmetric with respect to so .
Thus, it suffices to show . This is obvious because . Therefore we are done.
We will use complex numbers, with the circumcircle of as the unit circle. Let such that We claim that the circumcircle of passes through This is true if is real. This is true if We can compute so we are done.
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