let X1 and X2 be independent Poisson variables with respective parameters μ1 and μ2. Let S = X1 + X2. Is X1 given S=s a binomial dsitribution? What is the parameters? I just can show that S is a Poisson with mean μ1 + μ2. But im not confirm X1 given S is a binomial or not? Someone please help to prove it.