If I have got $\ X_1,...X_n $ all independent and identically distributed with the exponential distribution with parameter $\ 1/\theta$. Then how can I deduce that $\ S=X_1+...+X_n $ has the gamma distribution with paramters $\ (n,\theta)$? The previous question asks me to calculate the moment generating function of S ( I don't know if this is meant to help me do this part) and it says I can use the fact that $\int x^{n-1}e^{-\beta x}dx ~~ = ~~(n-1)!/\beta^n $
This is not a homework question but is a past paper question. Thanks