方差还有一个性质:若a为常系数,则D(aX)=(a^2)D(X), D(X+a)=D(X)
故
D(2X-Y)=D(2X)+D(-Y)=(2^2)D(X)+[(-1)^2]D(Y)=4D(X)+D(Y)
7. D(X)=36*(1/6)*(1-1/6)=5
D(Y)=9*(1/3)*(1-1/3)=2
D(X-Y+1)=D(X-Y)=D(X)+D(Y)=5+2=7
故
D(2X-Y)=D(2X)+D(-Y)=(2^2)D(X)+[(-1)^2]D(Y)=4D(X)+D(Y)
7. D(X)=36*(1/6)*(1-1/6)=5
D(Y)=9*(1/3)*(1-1/3)=2
D(X-Y+1)=D(X-Y)=D(X)+D(Y)=5+2=7
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