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MathNet 2017-part-01 — 176 problems (EN–VI) Question 52

Math Olympiad Vault · MathNet 2017-part-01 — 176 problems (EN–VI)

Question 52

Let $\triangle ABC$ be a triangle with $AB \lt AC$ inscribed into a circle $c$. The tangent of $c$ at the point $C$ meets the parallel from $B$ to $AC$ at the point $D$. The tangent of $c$ at the point $B$ meets the parallel from $C$ to $AB$ at the point $E$ and the tangent of $c$ at the point $C$ at the point $L$. Suppose that the circumcircle $c_1$ of the triangle $BDC$ meets $AC$ at the point $T$ and the circumcircle $c_2$ of the triangle $BEC$ meets $AB$ at the point $S$. Prove that the lines $ST, BC, AL$ are concurrent.