X-Y=5,X2+Y2=19,求xy的值

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X-Y=5,X2+Y2=19,求xy的值
X-Y=5,X2+Y2=19,求xy的值

X-Y=5,X2+Y2=19,求xy的值
(x-y)^2=x^2+y^2-2xy
5^2=19-2xy
-2xy=6
xy=-3

(X-Y)2=X2-2XY+Y2=19-2XY=25
所以XY=-3

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