L'Hopital's rule
L'Hôpital's Rule is a method in differential calculus for calculating the limit of a quotient of two functions wherein the entire expression approaches an indeterminate form of 0/0 or infinity/infinity. In the event that this is the case, the limit is equal to the limit of the quotient of the first derivatives of the two functions (provided that limit exists). Should this also yield an indeterminate form, the process is repeated until a meaningful result is obtained.[1]
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L'Hopital's Rule is not to be confused with the quotient rule, which allows for the calculation of the derivative of a single function that contains a quotient.
Examples
Example 1
A standard application of L'Hopital's rule is in evaluating the limit
In the preceding notation, this is the situation with 
and 
. Both the numerator and the denominator tend to 0 as 
tends to 0, i.e., 
, and so L'Hôpital's rule implies that
Example 2
L'Hopital's rule may also be used in the evaluation of the indeterminate form infinity/infinity. This version of the rule is useful in computing the horizontal asymptotes of rational functions. For example, suppose we seek to compute
This is an indeterminate form 
. Applying L'Hopital's rule once yields
This is still an indeterminate form. To evaluate the limit, it is necessary to invoke L'Hopital's rule a second time:
We conclude that
An easy extension of this argument is useful for finding horizontal asymptotes of more general rational functions. Suppose that 
and 
are two polynomials of equal degree 
. Applying L'Hopital's rule times we may discover that
where 
and 
are the leading coefficients of and (i.e., the coefficients on the term 
in these two polynomials). The example given is a case of this fact with 
(since both and are quadratic), and with 
and 
.
Example 3
We can use L'Hôpital's rule to prove the following:
This is another example where the limit is in the form of .
Proof
- For Integer n:
- For non-integer n:
- (where
is the floor function of n and 
is the ceiling function of n)
- Since
, 
and therefore
- This completes the proof.
