Question

the side of bc of ∆ abc is produced on both sides prove that the sum of the exterior angles so formed is greater than angle a by two right angles

Answer 1

Adding 1 & 2
b’ + c’ = a + c + a + b
b’ + c’ = a + (a + b + c)
b’ + c’ = a + (180) … (a + b + c) = 180
Since a is an internal angle of a triangle, it has to be a positive value.
From the figure we can say that
b’ = 180 - b = a + c ….. (1)
c’ = 180 - c = a + b ….. (2)