设随机变量X、Y满足D(X+Y)=D(X
设随机变量X、Y满足D(X+Y)=D(X-Y),则必有 A X与Y相互独立 B X与Y不相关 D（Y）=0 D D（X)D(Y)=0
1.D(X+Y)=D(X)+D(Y)+2cov(X,Y); ..D(X-Y)=D(X)+D(Y)-2cov(X,Y); 2.D(X+Y)=D(X-Y),cov(X,Y)=0,故B： X与Y不相关