Question

What is the reason for step 2 in this proof?

Given:

Angles A and B are supplementary.
The measure of angle A = 45º.

Find the measure of angle B. Proof:

1. Angles A and B are supplementary and angle A = 45º. (Reason: Given)

2. Measure of angle A + measure of angle B = 180º ( Reason: ?? )

3. 45º + measure of angle B = 180º (Reason: Substitution)

4. Measure of angle B = 135º (Reason: Subtraction Property of Equality)

A) Substitution Property of Equality
B) Definition of Supplementary Angles
C) Transitive Property of Equality

Answer 1

Angles A and B are said to be supplementary. This piece of information should be used. Supplementary angles are angles whose sum equals 180˚. This would be later used in the further steps to fin the unknown angle B.

Thus, the answer is B) Definition of Supplementary Angles