已知x^2+y^2-4x+2y+5=0,求(x^2-3xy+2y^2)/(x^2-y^2)的值
问题描述：
已知x^2+y^2-4x+2y+5=0,求(x^2-3xy+2y^2)/(x^2-y^2)的值
答
(x²-4x+4)+(y²+2y+1)=0
(x-2)²+(y+1)²=0
所以x-2=0,y+1=0
x=2,y=-1
原式=(x-y)(x-2y)/(x+y)(x-y)
=(x-2y)/(x+y)
=(2+2)/(2-1)
=4