2004 IMO Problems/Problem 5

Problem

In a convex quadrilateral $ABCD$ , the diagonal $BD$ bisects neither the angle $ABC$ nor the angle $CDA$ . The point $P$ lies inside $ABCD$ and satisfies $\angle PBC = \angle DBA \text{ and } \angle PDC = \angle BDA.$

Prove that $ABCD$ is a cyclic quadrilateral if and only if $AP = CP.$

Solution

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Let $K$ be the intersection of $AC$ and $BE$ , let $L$ be the intersection of $AC$ and $DF$ ,

$\angle PBC=\angle DBA$ , so $AD=CE$ , and $DE//AC$ . $\angle PDC=\angle BDA$ , so $AB=CF$ , and $AC//BF$ .

$\angle PLK=\frac12(\arc{AD}+\arc{CF}=\frac12(\arc{CE}+\arc{AB}=\angle PKL$ (Error compiling LaTeX. Unknown error_msg), so  is an isosceles triangle.
Since , so  and  are isosceles triangles. So  is on the angle bisector oof , since  is 
an isosceles trapezoid, so  is also on the perpendicular bisector of . So .

~szhangmath

2004 IMO (Problems) • Resources
Preceded by Problem 4	1 • 2 • 3 • 4 • 5 • 6	Followed by Problem 6
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2004 IMO Problems/Problem 5

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