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MathNet 2025-part-01 — 187 problems (EN–VI) Question 62

Math Olympiad Vault · MathNet 2025-part-01 — 187 problems (EN–VI)

Question 62

The angle bisectors of an acute triangle $ABC$ meet at point $I$. The line $AI$ meets the circumcircle of the triangle $ABC$ at point $D$ ($D \neq A$) and the side $BC$ at point $E$. The line $BI$ meets the circumcircle of the triangle $CDI$ at point $K$ whereas the line $CI$ meets the circumcircle of the triangle $BDI$ at point $L$ ($K \neq I, L \neq I$).

a. Prove that the line $DI$ is tangent to the circumcircle of the triangle $IKL$.

b. Prove that points $A, K, L, E$ are concyclic.