18.) Prove that all right angles are equal.

In paragraph proof...

Given: Angle A and angle B are congruent and supplementary.

Prove: Angle A and Angle B are right angles.

Angle A and angle B are congruent and supplementary, so the measure of angle A = the measure of angle B and A + B = 180°. Substituting the measure of angle A for the measure of angle B gives A + A = 180°. Therefore, the measure of angle A = 90°. Since angle A is congruent to angle B, the measure of angle B = 90°, as well. Thus both angles are right angles and are equal.

Another paragraph form...

Given: Angle A and Angle B are right angles.

Prove: Angle A is congruent to angle B.

Angle A and angle B are both right angles, so the measure of Angle A is 90° and the measure of angle B is 90°, due to the definition of right angles. Substituting the measure of angle A for the 90°, we find that the measure of angle A = angle B. Therefore, these right angles are equal, causing all right angles to be equal to each other and equal 90°.

This information was found in my Prentice Hall© Geometry Book.