If $X_i \sim Bernoulli(\theta)$, then the $\sum_1^n X_i \sim Binomial(n, \theta)$
I don't know how it is derived, could anyone show or prove it to me.
Besides, there are similar knowledges such as $X_i \sim Exp(1/\theta)$, then the $\sum_1^n X_i \sim Gamma(n,1/\theta)$ and I believe it is the Same method.
Many thanks !!