二维随机变量（X,Y）的概率密度为f(x,y)=12y² 当
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Cov(X,Y) = E{ [X-E(X)] [Y-E(Y)] }
= E{ XY - E(X)Y - E(Y)X +E(X)E(Y) }
= E(XY) - E(X)E(Y)
ρxy = Cov(X,Y)/[√D(X)√D(Y)]
= [E(XY) - E(X)E(Y)]/[√D(X)√D(Y)]
E(X) = ∫∫xf(x)dydx = 4/5
E(Y) = ∫∫yf(x,y)dydx = 3/5
E(X²) = ∫∫x²f(x,y)dydx = 2/3
E(y²) = ∫∫y²f(x,y)dydx = 2/5
E(XY) = ∫∫xyf(x,y)dydx = 1/2
D(X) = E(X²) - E²(X) = 2/75
D(Y) = E(Y²) - E²(Y) = 1/25
所以：
ρxy = [E(XY) - E(X)E(Y)]/[√D(X)√D(Y)]
= (1/2 - 12/25)/√(2/(75*25))
= (1/2)*√(3/2）
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