xy=m,且1/x2+1/y2=n,则（x-y)2=
问题描述：
xy=m,且1/x2+1/y2=n,则（x-y)2=
3y=x+2z,那么x2+9y2+4z2-6xy-12yz+4xz=
答
(x^2+y^2)/(xy)^2=n
x^2+y^2=m^2n
(x-y)^2=m^2n-2m