1、已知x^2+y^2-2x+4y+5=0,求4/(y+1/x)的值.
(x-1)^2+(y+2)^2=0
所以x-1=0,y+2=0
x=1 y=-2
y+1/x=-1
4/（y+1/x）=-4
2、已知x=1-a/1+a,y=3-2a/2-3a,用含x的代数式表示y.
x=(1-a)/(1+a)
x+xa=1-a
a=(1-x)/(1+x)
y=(2-3a)/(3-2a)
3y-2ay=2-3a
a=(2-3y)/(3-2y)
(1-x)/(1+x)=(2-3y)/(3-2y)
(1-x)(3-2y)=(2-3y)(1+x)
3-3x-2y+2xy=2-3y+2x-3xy
1+5xy=-y+5x
5xy+y=5x-1
y=(5x-1)/(5x+1)