Answer:
A
Step-by-step explanation:
The exterior angle is equal to the sum of the opposite interior angles.
Call the inside angle D
A+ B + D = 180 sum of the angles of a triangle is 180
C+D = 180 since the form a straight line
Solving for D
D = 180-C
Substitute this into the first equation
A+ B + 180-C = 180
Subtract 180 from each side
A+ B + -C =0
Add C to each side
A + B = C
This proves the theorem
