Question

Given triangle ABC
Prove angle 1 + angle 2 + angle 3 = 180

Answer 1

18) Construction
19) Alternate interior angles from parallel
20) Supplementary angles/straight angles
21) Substitution

Explanation:

18) Just creating a line for proofs
19) Since the lines are parallel, and the angles are on the opposite interior sides, you can use alternate interior angles to prove that the angles are the same.
20) All angles on a line must equal 180, so those 3 angles must add up to 180.
21) If mACD + m2 + mBCE = 180, and mACD = m1 and mBCE = m3, substitute, and you get m1 + m2 = m3