若x-y=3,x²+y²=17,xy=?
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![若x-y=3,x²+y²=17,xy=?](/uploads/image/z/1997707-67-7.jpg?t=%E8%8B%A5x-y%3D3%2Cx%26%23178%3B%2By%26%23178%3B%3D17%2Cxy%3D%3F)
若x-y=3,x²+y²=17,xy=?
若x-y=3,x²+y²=17,xy=?
若x-y=3,x²+y²=17,xy=?
(x-y)²
=x²+y²-2xy
=9
17-2xy=9
2xy=17-9
2xy=8
xy=4
4
(x-y)^2=9
x^+y^-2xy=9
代入求得2xy=8
xy=4
(x-y)^2=9
x^2+y^2-2xy=9
所以17-2xy=9
所以xy=4
(x-y)^2=x^2+y^2-2xy=3^2=9
所以xy=(17-9)/2=4