Let X∼Poisson(λ)X \sim \text{Poisson}(\lambda)X∼Poisson(λ). Find E[X(X−1)]E[X(X-1)]E[X(X−1)].
λ\lambdaλ
λ2\lambda^2λ2
λ2+λ\lambda^2 + \lambdaλ2+λ
λ/2\lambda/2λ/2