# Math Help - kindly explain shifting from integral to summation

1. ## kindly explain shifting from integral to summation

Can any body explain me second and third step of this solution.
How book moved from integral to summation

2. Hi

They used the expansion $\left(1-e^{-\frac{\gamma}{\bar{\gamma}}}\right)^{M-1} = \displaystyle\sum_{n=0}^{M-1} \binom{M-1}{n} \left(-1\right)^n e^{-n\frac{\gamma}{\bar{\gamma}}$

Then they have inverted the intergral and the sum and computed the integral