You can't. . . It is quite effective to use Robida's law to find the limit of indefinite form, but it is not omnipotent. if

lim(f'(x)/g'(x))

(x to 0 or infinity)

Does not exist and is not infinite, or

lim(f'(x)/g'(x))

(x to 0 or infinity)

When the limit is uncertain, L'H?pital's law will fail. Whether the original formula has a limit or not needs to be solved by another method. . . I can't type that score, sorry. This is the original text on page 97 of Higher Mathematics Policy of Higher Education Press.

Our teacher said so. There are two situations where Robida's law does not apply. One is that the score is meaningless at some points, and the other is that the limit of numerator or denominator is not easy to calculate.