Question

Prove that every rectangle is a parallelogram.

Answer 1

Let ABCD be a rectangle.
$\angle A=\angle B=\angle C=\angle D={90}^{\circ }.$
For any quadrilateral to be a parallelogram, pair of opposite angles should be congruent.
In rectangle ABCD,
$\angle A=\angle C={90}^{\circ }$
$\angle B=\angle D={90}^{\circ }$
Thus, rectangle ABCD is a parallelogram .