Question

Gordon is writing a proof of the theorem stated below.

Theorem: The sum of interior angles of a triangle is 180°.

He first drew the figure shown below.



Which theorem will he most likely use in the proof?
Answer
A linear pair of angles is supplementary.
Supplements of congruent angles are congruent.
If two angles of a triangle are congruent, the sides opposite these are congruent.
If two sides of a triangle are congruent, the angles opposite these sides are congruent.