Is This a Parallelogram?

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If both pairs of opposite sides of a quadrilateral are congruent, is the quadrilateral a parallelogram? This task asks learners to determine the answer and to support their answer with a proof. The resource includes a commentary for teachers that gives suggestions about how to guide students through two different methods of proof. One method uses congruent triangles to establish congruent alternate interior angles to show that opposite sides are parallel, and the other method uses a rigid motion argument.