Angle Sum Property of a Triangle Theorem 1: Prove that sum of all three angles is 180° or 2 right angles. Given: ∆ABC To prove: ∠A + ∠B + ∠C = 180° Construction: Draw PQ || BC, passes through point A.Proof: ∠1 = ∠B and ∠3 = ∠C ....... (i) [∵ alternate angles ∵ PQ || BC] ∵ PAQ is a line ∴∠1 + ∠2 + ∠3 = 180° (linear pair application) ∠B + ∠2 + ∠C = … [Read more...] about How to Prove the Angle Sum Property of a Triangle