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.