Take a look here:
http://www.algebra.com/algebra/homework/Geometry-proofs/Geometry_proofs.faq.question.392003.html
Write a proof.
1. Prove: If the diagonals of a parallelogram are perpendicular, then the parallelogram is a rhombus.
2. Prove: If the diagonals of a parallelogram are congruent, then parallelogram is a rectangle.
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