Question

Using opposite angles test for parallelogram, prove that every rectangle is a parallelogram.

Answer 1

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