1987 OIM Problems/Problem 6
Problem
Let 
be a planar convex quadrilateral, 
and 
are points of 
and 
respectively such that:
![\[\frac{AP}{PD}=\frac{AB}{DC}=\frac{BQ}{QC}\]](http://latex.artofproblemsolving.com/5/f/d/5fd51a80cf4d97e0022bf5bddfb94e875038c495.png)
Prove that the angles that are formed between line 
and lines 
and 
are equal.
Solution
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