f(x) = (x+1)/(x-1) = (x-1+2)/(x-1) = 1 + 2/(x-1)
x趋近于1+时,x-1趋近于0+,2/(x-1)趋近于+∞,f(x) 趋近于+∞；
x趋近于1-时,x-1趋近于0-,2/(x-1)趋近于-∞,f(x) 趋近于-∞