Polish MO finals, problem 3
Source:
April 10, 2008
trigonometrygeometry proposedgeometry
Problem Statement
In a convex pentagon in which BC\equal{}DE following equalities hold:
\angle ABE \equal{}\angle CAB \equal{}\angle AED\minus{}90^{\circ},\qquad \angle ACB\equal{}\angle ADE
Show that is a parallelogram.