1) Suppose a,b such that,
g(a)=g(b) since f is an one to one function.
a=b since g is an one to one function.
Hence is an one to one function.
2) Supose g(a) = c and f(c) = b,
Since is one to one it is invertible,
Since f and g are one to one they are invertible.
From A and B,