According to the equation
Lets suppose that
X and Y are two circle touch each other at P
AB is the common tangent to circle X and Y at point A and B.
According in the given figure, In triangle
PAC , ∠CAP = ∠APC = α
similarly
CB = CP , ∠CPB = ∠PBC = β
Now triangle APB,
∠PAB + ∠PBA + ∠APB = 180
α + β + (α + β) = 180
2α + 2β = 180
α + β = 90
∴ ∠APB = 90 = α + β
This is the required solution