用导数基本法则：(x^n)'=n*[x^(n-1)]
商法则：(u/v)'=(vu'-uv')/v^2
y=x/(x-2)
y'=[(x-2)(x)'-x(x-2)']/(x-2)^2
=(x-2-x)/(x-2)^2
=-2/(x-2)^2