I know that E[Y2] is equal to E[Y]  2 but you need the compute the double integral of xy*f(xy) to compute E[XY] what happens to the 2? what about E[A(A+3)], where A is a random variable?

I know that E[Y2] is equal to E[Y]  2 but you need the compute the double integral of xy*f(xy) to compute E[XY] what happens to the 2? what about E[A(A+3)], where A is a random variable?
X(Y2) = XY  2X, therefore E(X(Y2)) = E(XY) 2E(X).
E(A(A+3)) = E(A^2) + 3E(A)
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