当n→+∞时,n→+∞,ln n →+∞
所以是(+∞)/(+∞)型的未定式.
利用罗比达法则,有：lim n→+∞ (n/ln n) = lim n→+∞ (n')/(ln n)' = lim n→+∞ [1/(1/n)] = lim n→+∞ (n) = +∞