已知DX=25,DY=36,ρ=0.4,求D(X+Y),D(X-Y),D(2X+3Y)
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已知DX=25,DY=36,ρ=0.4,求D(X+Y),D(X-Y),D(2X+3Y)
协方差COV(X,Y)=根号(DX)*根号(DY) *0.4=5*6*0.4=12
D(X+Y)=DX+DY+2*COV(X,Y)=25+36+2*12=85
D(X-Y)=Dx+Dy-2*COV(X,Y)=25+36-2*12=37
协方差COV(2X,3Y)=2*3*COV(X,Y)=72
D(2X+3Y)=4DX+9DY-2*COV(2X,3Y)=-440
注:设C, a, b=Const, DCX=C^2DX , COV(aX,bY)=a*b*COV(X,Y)
D(X+Y)=DX+DY+2*COV(X,Y)=25+36+2*12=85
D(X-Y)=Dx+Dy-2*COV(X,Y)=25+36-2*12=37
协方差COV(2X,3Y)=2*3*COV(X,Y)=72
D(2X+3Y)=4DX+9DY-2*COV(2X,3Y)=-440
注:设C, a, b=Const, DCX=C^2DX , COV(aX,bY)=a*b*COV(X,Y)
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