What else can you prove about quadrilaterals? Prove that if a pair of opposite sides of a quadrilateral is both congruent and parallel, then the quadrilateral must be a parallelogram. For example, for the quadrilateral
If a pair of opposite sides is parallel and congruent then the quadrilateral is a parallelogram.
Use the if part of the statement as the givens, and the then part of the statement as the prove portion in the proof.
Segment is congruent to itself
Alt. int. angles