Prove that the exterior angle of a triangle equals sum of two interior opposite angles. (NCERT Class 8/9)

Answer: In △ABC, extend BC to D. Exterior angle ∠ACD = 180°−∠C. But ∠A+∠B+∠C=180°. So 180°−∠C=∠A+∠B. Hence exterior angle equals sum of opposite interior angles.