Prove a Quadrilateral is a Parallelogram by one of the following
Definition: The quadrilateral has 2 pairs of parallel sides. (same slope!)
Theorem 1: The opposite sides are congruent
Theorem 2: The opposite angles are congruent
Theorem 3: The diagonals bisects each other.
Theorem 4: The consecutive angles are supplementary.
Theorem 5: One pair of sides are both parallel (same slope) and congruent.