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MathNet 2023-part-02 — 191 problems (EN–VI) Question 136

Math Olympiad Vault · MathNet 2023-part-02 — 191 problems (EN–VI)

Question 136

Given an acute $\triangle ABC$ with $AB \gt AC$, let $M$ be the midpoint of the side $BC$. Let $I$ be the incentre of $\triangle ABC$, and let $D$ be the foot of the perpendicular from $I$ to $BC$. The line passing through $A$ and perpendicular to $BC$ meets the circumcircle of $\triangle ABC$ again at $E \neq A$. Suppose $\angle MID = \angle ACB - \angle ABC$. Prove that $DE = DI$.