Let be holomorphic, let and let be such that the disk then .
I started from the Cauchy integral formula and got .
I feel like something is missing in the question.
We want to show
Cauchy integral formula is
Let and .
Then, since f is holomorphic, and from that we get that .
Now we integrate around the upper half of the circle from 0 to and around the lower half of the circle from 0 to . For the second we might as well integrate 0 to but change the sign of .
I may have made mistakes though.