Two-Column Proofs Practice Tool. Select a proof from the list below to get started. To see and record your progress, log in here. Title Difficulty Solved By Date Added; Complementary Angles 1: easy : 218 (65%) 2008-12-27 ; Complementary Angles 2: easy : 172 (51%) 2008-12-27 ; Supplementary Angles 1: easy : 207 (62%). The theorem CPCTC tells that when two triangles are congruent then their corresponding sides and angles are also said to be congruent. For example, triangle ABC and triangle PQR are congruent triangles therefore according to the theorem the sides AB = PQ, BC = QR, and CA = RP. Also ∠A = ∠P, ∠B = ∠Q, and ∠C = ∠R.