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6. Let X ~ Poisson( )). Find E[X(X -1)(X - 2)] using the following steps: . Write using definition E[X(X - 1) (X - 2)] = Er k(k - 1) (k - 2) P(X = k) . Change the summation to start from k = 3, because k(k - 1) (k - 2) = 0 when k = 0, 1, 2 . Rewrite the summation using a new index i = k - 3 . Simplify the summation using the identity 2 ! Hence, find E(X3)