令M=X2Y1-X1Y2
则M^2+ (X1Y1+X2Y2)^2
=(X2Y1)^2 + (X1Y2)^2 + (X1Y1)^2 + (X2Y2)^2
=(X1^2+X2^2)(Y1^2+Y2^2)
所以M^2=(X1^2+X2^2)(Y1^2+Y2^2)- (X1Y1+X2Y2)^2
即可求得M的值
已知X1^2+X2^2,X1+X2,Y1^2+Y2^2,Y1+Y2,X1Y1+X2Y2求X2Y1-X1Y2
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