In order to prove that the sum of angles of a triangle is 180, you must know the theorems of angles of a triangle.We know that, alternate interior angles are of equal magnitude.This\’ll help us get the answer. Let us assume a triangle ABC. Now we have to substitute the angles.∠PAB + ∠BAC + ∠CAQ = 180°. where PQ line parallel to side BC touching vertex A.∠PAB = ∠ABC & ∠CAQ = ∠ACB BECAUSE alternate interior angles are congruent..Hence we get, ∠ABC + ∠BAC + ∠ACB = 180° [Proved]