Theorem : If a side of a triangle is produced, then the exterior angle so formed is equal to the sum of the two interior opposite angles.
Given : A ▲ABC whose side BC has been produced to D forming exterior angle ∠4 To Prove : ∠4 = ∠1 + ∠3 Proof : We have, ∠1 + ∠2 + ∠3 = 180° ( Sum of the angles of a triangle) Also, ∠2 + ∠4 = 180° ( Linear pair) ∴ ∠2 + ∠4 = ∠1 + ∠2 + ∠3 Hence, ∠4 = ∠1 + ∠3.