Difference between revisions of "2007 IMO Shortlist Problems/G3"

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==IMO 2007 Short list==
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==IMO 2007 Short list/G3==
 
[[File:Trapezoid 17.png|500px|right]]
 
[[File:Trapezoid 17.png|500px|right]]
 
The diagonals of a trapezoid <math>ABCD</math> intersect at point <math>P.</math>
 
The diagonals of a trapezoid <math>ABCD</math> intersect at point <math>P.</math>

Latest revision as of 13:33, 2 March 2023

IMO 2007 Short list/G3

Trapezoid 17.png

The diagonals of a trapezoid $ABCD$ intersect at point $P.$

Point $Q$ lies between the parallel lines $BC$ and $AD$ such that $\angle AQD = \angle CQB,$ and line $CD$ separates points $P$ and $Q.$

Prove that $\angle BQP = \angle DAQ.$

Proof

$\angle AQD = \angle CQB \implies$

$BQ$ and $AQ$ are isogonals with respect $\angle CQD.$

$P =AC \cap BD, BC || AD \implies$

$QS || AD$ is isogonal to $QP$ with respect $\angle CQD.$

From the converse of The isogonal theorem we get

$\angle BQP = \angle SQA = \angle DAQ.$ $\blacksquare$

vladimir.shelomovskii@gmail.com, vvsss