1、3x/（x-3）²+x/x-3
=[3x+x(x-3)]/(x-3)²
=(3x+x²-3x)/(x-3)²
=x²/(x-3)²
2、（1+1/x-2）÷x²-1/2x-4
=[(x-1)/(x-2)]×[2(x-2)/(x+1)(x-1)]
=2/(x+1)
3、若1/x-1/y=2,求3x-2xy-3y/x-2xy-y的值
1/x-1/y=(y-x)/xy=2
y-x=2xy
x-y=-2xy
原式=[3(x-y)-2xy]/[(x-y)-2xy]
=(-6xy-2xy)/(-2xy-2xy)
=(-6-2)/(-2-2)
=2