Criteria for a Parallelogram
Big Idea: We already know that a parallelogram has special properties (opposite sides parallel, opposite sides equal, opposite angles equal, diagonals bisect each other). But what if we want to prove that a quadrilateral IS a parallelogram? That's where the criteria come in — they are the sufficient conditions that guarantee a quadrilateral is a parallelogram.
📐 Definition Recap
A parallelogram is a quadrilateral in which both pairs of opposite sides are parallel.
$$\text{If } ABCD \text{ is a parallelogram, then } AB \parallel CD \text{ and } AD \parallel BC.$$