Online Geometry Problem 831: Quadrilateral, Diagonal,
Midpoint, Opposite Angles, Congruence, Parallelogram. Level: High School,
SAT Prep, College,
Mathematics Education

The figure below shows a quadrilateral ABCD
with angle BAD = angle BCD and O midpoint of diagonal BD. Prove that
ABCD is a parallelogram. Problem submitted by Jacob Ha, Keith Law
....