A ,46
D(x+y) = DX + DY +2 Cov(X,Y)
= 25 + 9 + 2 * 0.4 * 5 * 3
= 34 + 12
= 46
设DX=25,DY=9,Pxy=0.4,求D(x+y)=（ ）
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