The sum of two angles of a triangle is equal to its third angle. Determine the measure of the third angle.

Let,

∠1, ∠2 and ∠3 be the angles of a triangle.

∠1 + ∠2 = ∠3 (Given) (i)

We know that,

∠1 + ∠2 + ∠3 = 180^o

∠3 + ∠3 = 180^o [From (i)]

2∠3 = 180^o

∠3 = 90^o

Thus, third angle is 90^o.