1. The problem statement, all variables and given/known data X and Y are joointly distributed discrete random variables with probability mass function pX,Y(x,y)=(c/ec)(1-c)xcy/y!, x,y=0,1,2,..., 0<c<1 Find E(XY) 2. Relevant equations 3. The attempt at a solution Code (Text): By definition, E(X[SUP]Y[/SUP]) ∞ ∞ = ∑ ∑ x[SUP]y[/SUP] (c/e[SUP]c[/SUP])(1-c)[SUP]x[/SUP]c[SUP]y[/SUP]/y! x=0 y=0 How can we calculate this double sum? We can pull out the constant c/ec, but what's next? Thanks for any help!