# 2012 IMO Problems/Problem 5

Lets draw an circumcircle around triangle ABC (= circle *a*), a circle with it's center as A and radius as AC (= circle *b*),
a circle with it's center as B and radius as BC (= circle *c*).
Since the center of *a* lies on the line BC the three circles above are coaxial to line CD.
Let ) Line AX and Line BX collide with *a* on P (not A) and Q (not B). Also let R be the point where AQ and BP intersects.
Then since angle AYB = angle AZB = 90, by ceva's theorem in the opposite way, the point R lies on the line CD.

Since triangles ABC and ACD are similar, AL^2 = AC^2 = AD X AB, so angle ALD = angle ABL
In the same way angle BKD = angle BAK
So in total because angle ARD = angle ABQ = angle ALD, (A, R, L, D) is concyclic
In the same way (B, R, K, D) is concyclic
So angle ADR = angle ALR = 90, and in the same way angle BKR = 90 so the line RK and RL are tangent to each *c* and *b*.
Since R is on the line CD, and the line CD is the concentric line of *b* and *c*, the equation RK^2 = RL^2 is true.
Which makes the result of RK = RL. Since RM is in the middle and angle ADR = angle BKR = 90,
we can say that the triangles RKM and RLM are the same. So KM = LM.

Edit: I believe that this solution, which was posted on IMO 2012-4's page, was meant to be posted here.