Answer:
Given
This is our initial premise.
2) Linear pairs of angles are supplementary
This one is a little questionable, as some definitions of linear pairs require supplementary angles, whereas others only require the intersection of two lines. Check your book or notes for any given theorems regarding supplementary angles.
3)
m
∠
A
B
C
+
m
∠
C
B
D
=
180
∘
The definition of supplementary angles is that two angles are supplementary if their measures sum to
180
∘
.
4) Substitution of 1. into 3.
As with 2), this may differ based on the teacher or book. Some may prefer that you write out the equation, whereas others may be satisfied with the references as given. Check for similar examples.
5)
m
∠
A
B
C
=
90
∘
Subtracting
90
∘
from each side of 4. gives us the above result.
6) Definition of right angle
Explanation: