Prove that the sum of angles in a triangle is 180°. (NCERT Class 7/9 – Fundamental)

Answer: In △ABC, draw DE ∥ BC through A. Then ∠DAB=∠ABC and ∠EAC=∠ACB (alternate angles). On line DE, ∠DAB+∠BAC+∠EAC=180°. Substitute to get ∠A+∠B+∠C=180°.