# When to Use L'Hôpital's Rule

The textbook explanation shows that L'Hôpital's rule can be used on a rational function ${f(x)}\over {g(x)}$ if it is continuous, and $\lim_{x \to c}f(x) = \lim_{x \to c}g(x) = 0$ or $\pm \infty$, and $g'(x) \neq 0$.

The textbook doesn't explain, however, what steps are necessary to take before I dive blindly into applying L'Hôpital's rule e.g. is there a more elegant way to solve the limit?

Usually I would assume that ${f(x)}\over {g(x)}$ is in an irreducible form and the limit still evaluates to $0 \over 0$ or $\infty \over \infty$. If I can't find any terms to cancel out, then I would proceed to use L'Hôpital's rule ...

I guess what I'm asking is what is the best approach to any limit problem? How do I know when I should break out L'Hôpital's rule?

• Uhh, have you checked the provided proof?
– Yes
Oct 27, 2015 at 7:04
• L'Hôpital's rule saves time and helps in easily evaluating imits. But if you want elegant proofs, then this will be your last choice and the conventional method will suit you better. Oct 27, 2015 at 7:04
• Not more elegant, but easier to handle (esp. if several rounds of l'Hôpita are needed) is to use Taylor expansions ... Oct 27, 2015 at 7:13
• The post seems to be specifically about rational functions. If top and bottom have limit $0$ at $c$, then $x-c$ divides both, so in principle one can do the division. It seems harmless to use L'Hospital to bypass division. Oct 27, 2015 at 7:14
• I tell students that l'Hôpital's theorem is something like the Paris gun: a very powerful weapon, but that can do damage for no good. Oct 27, 2015 at 9:39

Let's suppose we are dealing with limits of the form $x \to a$. Then we have two cases where L'Hospital's Rule is applicable.
Theorem 1: Let $f, g$ be function differentiable in a deleted neighborhood of $a$. If $f(x) \to 0, g(x) \to 0$ as $x \to a$ and $f'(x)/g'(x) \to L$ as $x \to a$ then $f(x)/g(x) \to L$ as $x \to a$.
This is the typical case of indeterminate form $0/0$. Note that the result is valid even if $L$ above is infinite.
Theorem 2: Let $f, g$ be function differentiable in a deleted neighborhood of $a$. If $|g(x)| \to \infty$ as $x \to a$ and $f'(x)/g'(x) \to L$ as $x \to a$ then $f(x)/g(x) \to L$ as $x \to a$.
This is the case where denominator $g(x)$ tends to infinity. There is no need to check the behavior of numerator here.