21. Let X be a random variable. Prove that Var(X) = min t E 4 (X t)2 5 . Hint: Let = E(X)

21. Let X be a random variable. Prove that Var(X) = min t E 4 (X − t)2 5 . Hint: Let µ = E(X) and look at the expansion of E 4 (X − t)25 = E 4 (X − µ + µ − t)25 . B