x^2-y^2=18 ,x-y=6 ,则xy=?
问题描述:
x^2-y^2=18 ,x-y=6 ,则xy=?
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