x^2-y^2=18 ,x-y=6 ,则xy=?
人气:418 ℃ 时间:2020-02-03 17:46:06
解答
x^2-y^2=(x+y)(x-y)=6(x+y)=18
x+y=3 平方
x^2+2xy+y^2=9
x-y=6 平方
x^2-2xy+y^2=36
相减得
4xy=-27
xy=-27/4
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