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SMO 2026 Junior Round 2 Question 1

SMO · SMO 2026 Junior Round 2

Question 1

Let $ABCD$ be a rhombus and $E$ be a point on the diagonal $AC$. Let $F$ be the point on the side $BC$ such that $BF=DE$. Prove that
$$(AB+BF)\cdot FC=AE\cdot EC.$$