已知x-y/xy=2,求分式7x-5xy-7y/5x-5y
解,得：
x-y/xy=2
x-y=2xy
由7x-5xy-7y/5x-5y得,
==7x-5xy-7y/5(x-y)
==7(x-y)-5xy/10xy
==14xy-5xy/10xy
==9xy/10xy
==9/10
==0.9