1.已知x-3y=0,且xy≠0,求x²-y²/x²-xy+y²的值.

1.已知x-3y=0,且xy≠0,求x²-y²/x²-xy+y²的值
x-3y=0
x=3y
(x²-y²)/(x²-xy+y²)=(9y²-y²)/(9y²-3y²+y²)=8/7
3.当x取何值时,分式(1+2x)(1/2-x)/(2x-1)²的值为零.
(1+2x)(1/2-x)=0
2x-1≠0
∴x=-1/2
2.已知a/b=2,求a²-4ab+3b²/a²-8ab+7b²的值.
a/b=2
∴a=2b
(a²-4ab+3b²)/(a²-8ab+7b²)
=(4b²-8b²+3b²)/(4b²-16b²+7b²)
=1/5

1）8/7
2）-1/2
3）1/5