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

Opposite angle property of parallelogram says that the opposite angles of a parallelogram are congruent.

Given a rectangle which had at least one angle as 90°.

If A is 90° and AD = BC (opposite sides of rectangle are and =)

AB is transversal

A + B = 180 (angles on the same side of transversal is 180°)

But B + C is 180 (AD BC, opposite sides of rectangle)

A = C = 90°

Since opposite s are equal this rectangle is a parallelogram too.

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