From above figure , we can see that
Join B and D and produce BD to E
Then, p + q = β and s + t = x
We know that , Exterior angle = Sum of two interior angle
Now, s = p + α (Exterior angle of a Δ prop.)
Similarly, t = q + γ (Exterior angle of a Δ prop.)
Adding, s + t = p + q + α + γ
Putting s + t = x and p + q = β ,
Hence x = α + β + γ
