X+Y=8 ==> Y=8-X,
B(10,0.6),
EX=0*C0.6^0*0.4^10+1*C0.6^1*0.4^9+2*C0.6^2*0.4^8+...
+10*C0.6^10*0.4^0=10*0.6=6,
EX^2=EX(X-1)+EX=10(10-1)0.6^2+6=32.4+6=38.4,
DX=EX^2-(EX)^2=38.4-36=10*0.6*0.4=2.4,
EY=E(8-X)=8-EX=8-6=2,
EY^2=E(8-X)^2=E(8^2-16X+X^2)=E64-16EX+EX^2=64-16*6+38.4=6.4,
DY= EY^2-(EY)^2=6.4-4=2.4
或 DY=D(8-X)=D[8+(-X)]=D8+D(-X)=0+[(-1)^2]DX=DX=2.4