设随机变量X与Y相互独立,且E(X)=E(Y)=1,D(X)=2,D(Y)=3,试求（1）D(X-Y) （2）D(XY)

X,Y是两个相互独立的随机变量,则D(X-Y)=D(X)+(-1)^2*D(Y)=5
D(X)=E(X^2)-[E(X)]^2
E(X^2)=2+1=3
同理E(Y^2)=3+1=4
而cov(X,Y)=0,E[(X-E(X))(Y-E(Y))]=0
E(XY)=E(X)E(Y)=1
同理E(X^2*Y^2)=E(X^2)E(Y^2)=12
D(XY)=E(X^2*Y^2)-[E(XY)]^2=11