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.
![Image](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiPjxnTUPiNZUlIFCSoqJ33Iy6ydqXJ6qxPwRCY4dP6fekwKxqbfYjBhd0spZMurux0Xe0a_BSKSD6cPUkdsm1EsevJiL1QD_WEioB_EVk4DwzZNvThDh4cleoisHv2U3b5KH4_RXBS9c0/s320/Exterior+angle+prop.jpg)
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.