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A parallelogram has 2 sets of parallel sides. The opposite sides  are equal in length and opposite angles are equal. Examples of  parallelograms are squares, rectangles and â¦ rhombuses. You can find  the perimeter of a parallelogram by doubling the base+side length. 6 people found this useful.
Theorem 5: If the diagonals of a parallelogram are congruent, then the parallelogram is a rectangle. Proof: Let the parallelogram be ABCD with congruent diagonals AC and BD. Consider the overlapping triangles ADC and BCD. Since opposite sides of a parallelogram are congruent, AD = BC.