E(X) = (-2)*0.4 + 0*0.2 +2*0.4 = 0; E(X^2) = D(X) +[E(X)]^2=4+ 0^2 = 4; D(X+Y) = D(X) + D(Y) +2*CovX, Y)=1+1+2*0.5 = 3注：X^2表示X的平方
10：E(x)=-2×0.4+0×0.2+2×0.4=0
12: 1.5
