Interior and exterior angles

Exterior and interior angles are supplementary

Properties of Parallelograms  Opposite sides are congruent  Opposite angles are congruent  Consecutive angles are supplementary  Diagonals bisect each other

How to prove a quadrilateral is a parallelogram We will have 5 ways to do this: Both pair of opposite sides parallel Both pair of opposite sides congruent Both pair of opposite angles congruent One pair of sides both parallel and congruent Diagonals bisect each other

Summary of ways to prove a quadrilateral is a parallelogram