EX=4，DX=0.8
EY=4，DY=4
D(X+Y)=DX+DY+2Cov(X,Y)=3.6
∴Cov(X,Y)=-0.6
ρ=Cov(X,Y)/（DXDY)^(1/2)=-0.6/1.789=-0.335
