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MathNet 2012 Question 79

Math Olympiad Vault · MathNet 2012

Question 79

Let $ABC$ be a triangle with median $AK$. Let $O$ be the circumcenter of the triangle $ABK$.

a) Prove that if $O$ lies on a midline of the triangle $ABC$, but does not coincide with its endpoints, then $ABC$ is a right triangle.

b) Is the statement still true if $O$ can coincide with an endpoint of the midsegment?