The sum of interior angles of a triangle is equal to 180^o. This helps us to find one angle when others are given.

Ex: find the missing angle given the following figures

1.     50^o                                                                                                                a

     70^o                                x                             50^o                                                                                                  40⁰

Solution: a) sum of interior angles in a triangle = 180

Therefore, 50 + 70 + x = 180, x = 180 – 120 = 60^o

b) a + 50 +40 = 180, a = 180 - 90 =b) a + 50 +40 = 180, a = 180 - 90 = 90