x+1/x=3,求分式x²+1/x²的值
两边平方.∴x²+1/x²=9-2=7
2 已知1/x-1/y=5,求分式xy/y-x的值
解xy/（y-x）的倒数=1/x-1/y
∴xy/（y-x）=1/5