Question

1. fill in the \???\ section in the proof below.

given: line ab is perpendicular to bc.
proof: ∠1 is complementary to ∠2.

proof:

statements	reason
angle b is a right angle.	definition of a perpendicular line.
m∠b = 90	???
∠1 and ∠2 are adjacent	definition of adjacent angles
m∠1 < 90 and m∠2 < 90	definition of acute angles
∠1 and ∠2 are complementary	definition of complementary angles

Explanation:

Step1: Identify right angle measure rule

A right angle has a measure of 90 degrees by definition.

Step2: Match statement to reason

The statement $m\angle B = 90$ follows from the fact that $\angle B$ is a right angle, so the reason is the definition of a right angle.

Answer:

Definition of a right angle.