Question: prove that the opposite angles of a parallelogram are congruent?
Answer: THEOREM: If a quadrilateral is a parallelogram, it has 2 sets of opposite angles congruent. THEOREM: If a quadrilateral is a parallelogram, it has consecutive angles which are supplementary. ... THEOREM: If a quadrilateral has one set of opposite sides which are both congruent and parallel, then it is a parallelogram.