In order to derive the integration formula in question, we integrate the function around the closed rectangular path shown in the figure

According to the Cauchy-Goursat theorem, then

We now let , using the known integration formula

as

In order to derive the integration formula in question, we integrate the function around the closed rectangular path shown in the figure

According to the Cauchy-Goursat theorem, then

We now let , using the known integration formula

as