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Prove a angular equality

Source: Bulgaria TST 2003 P5

September 8, 2012
Asymptotegeometrytrigonometrytrig identitiesLaw of Sinesgeometry proposed

Problem Statement

Let ABCDABCD be a circumscribed quadrilateral and let PP be the orthogonal projection of its in center on ACAC. Prove that ∠APB=∠APD\angle {APB}=\angle {APD}
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