If the sides of a triangle are produced then the sum of the exterior angles i.e., ∠ a + ∠ b + ∠ c is equal to :

From above figure , we have
Since every exterior angle is equal to sum of interior opposite angles,
So, ∠ a = A + B, ∠ b = B + C and ∠ c = A + C
We know that , A + B + C = 180°
∴ ∠a +∠b + ∠c = 2(A + B + C) = 2 × 180° = 360°.