In calculus (an area of mathematics), **L'Hôpital's rule** uses derivatives to determine otherwise hard to compute limitss. If you are trying to determine the limit of some quotient *f*(*x*)/*g*(*x*), and both the numerator and denominator approach 0 or infinity, then differentiate numerator and denominator and determine the limit of the quotient of the derivatives. If that limit exists, the rule states that it will be the same as the original limit.

That is,

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2 Proof 3 Of interest |

## Examples

The a case of "0/0":

*Analyse des infiniment petits pour l'intelligence des lignes courbes*(1692), the first textbook to be written on the differential calculus.

## Proof

The proof of L'Hôpital's rule depends on Cauchy's mean value theorem.

According to Cauchy's mean value theorem there is a constant in the interval such that:

Since , we can say that:

If we let , we get:

Therefore

Q.E.D

## Of interest

Although the l'Hopital's rule is a powerful way of computing otherwise hard to compute limits, it is not always the easiest. Some special type of limits are actually easier to compute using the Taylor series expansion.E.g.