Difference between revisions of "L'Hopital's rule"
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− | '''L'Hôpital's Rule''' is a method in | + | '''L'Hôpital's Rule''' is a method in differential [[calculus]] for calculating the [[limit_(mathematics)|limit]] of a [[quotient]] of two [[function]]s 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 [[derivative]]s 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.<ref>http://mathworld.wolfram.com/LHospitalsRule.html</ref> |
{| style="border:1px solid #ccc;margin:auto" align="center" | {| style="border:1px solid #ccc;margin:auto" align="center" | ||
|+ '''Math form''' | |+ '''Math form''' | ||
| | | | ||
− | :<math>g'(x)\ | + | :<math>g'(x)\not\equiv 0</math> |
:C is some number such that | :C is some number such that | ||
::<math>f(c)=0</math> | ::<math>f(c)=0</math> | ||
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== Examples == | == Examples == | ||
+ | ===Example 1=== | ||
A standard application of L'Hopital's rule is in evaluating the limit | A standard application of L'Hopital's rule is in evaluating the limit | ||
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</math> | </math> | ||
+ | ===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 [[asymptote|asymptotes]] of rational functions. For example, suppose we seek to compute | 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 [[asymptote|asymptotes]] of rational functions. For example, suppose we seek to compute | ||
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::<math>\lim_{x \to \infty} \frac{2x^2+3x+2}{x^2-5x+7} = 2.</math> | ::<math>\lim_{x \to \infty} \frac{2x^2+3x+2}{x^2-5x+7} = 2.</math> | ||
− | An easy extension of this argument is useful for finding horizontal asymptotes of more general rational functions. Suppose that <math>f</math> and <math>g</math> are two polynomials of equal degree <math>n</math>. Applying L'Hopital's rule <n> times we may discover that | + | An easy extension of this argument is useful for finding horizontal asymptotes of more general rational functions. Suppose that <math>f</math> and <math>g</math> are two polynomials of equal degree <math>n</math>. Applying L'Hopital's rule <math>n</math> times we may discover that |
::<math>\lim_{x \to \infty} \frac{f(x)}{g(x)} = \frac{f_n}{g_n},</math> | ::<math>\lim_{x \to \infty} \frac{f(x)}{g(x)} = \frac{f_n}{g_n},</math> | ||
− | where <math>f_n</math> and <math>g_n</math> are the leading coefficients of <math>f</math> and <math>g</math> (i.e., the coefficients on the term <math>x^n</math> in these two polynomials). The example given is a case of this fact with <math>n=2</math> (since both <math>f</math> and <math>g</math> are quadratic), and with <math>f_n = 2</math> and < | + | where <math>f_n</math> and <math>g_n</math> are the leading coefficients of <math>f</math> and <math>g</math> (i.e., the coefficients on the term <math>x^n</math> in these two polynomials). The example given is a case of this fact with <math>n=2</math> (since both <math>f</math> and <math>g</math> are quadratic), and with <math>f_n = 2</math> and <math>g_n = 1</math>. |
+ | |||
+ | ===Example 3=== | ||
+ | We can use L'Hôpital's rule to prove the following: | ||
+ | |||
+ | :<math>\lim_{x\rightarrow\infty} \frac{x^n}{e^x}=0 \quad\forall n< \infty</math> | ||
+ | |||
+ | This is another example where the limit is in the form of <math>\infty/\infty</math>.<br> | ||
+ | '''Proof''' | ||
+ | :For Integer n: | ||
+ | ::<math>\forall n< \infty\quad\lim_{x\rightarrow\infty} \frac{x^n}{e^x}= | ||
+ | \lim_{x\rightarrow\infty} \frac{nx^{n-1}}{e^x}= | ||
+ | \lim_{x\rightarrow\infty} \frac{n\left(n-1\right)x^{n-2}}{e^x}= \cdots= | ||
+ | \lim_{x\rightarrow\infty} \frac{n!}{e^x}= 0</math> | ||
+ | :For non-integer n: | ||
+ | :::<math>\forall n< \infty\quad\lim_{x\rightarrow\infty} \frac{x^n}{e^x}= | ||
+ | \lim_{x\rightarrow\infty} \frac{nx^{n-1}}{e^x}= | ||
+ | \lim_{x\rightarrow\infty} \frac{n\left(n-1\right)x^{n-2}}{e^x}= \cdots= | ||
+ | \lim_{x\rightarrow\infty} \frac{\left[n \left(n-1 \right)\left(n-2 \right)\cdots \left(n-\lfloor n \rfloor \right) \right]x^{n-\lceil n \rceil}}{e^x}</math> | ||
+ | :::(where <math>\lfloor n \rfloor</math> is the [[floor function]] of n and <math>\lceil n \rceil</math> is the [[ceiling function]] of n) | ||
+ | ::Since <math>n-\lceil n \rceil <0</math>, <math>\lim_{x\rightarrow\infty}x^{n-\lceil n \rceil}=0</math> and therefore | ||
+ | :::<math>\lim_{x\rightarrow\infty} \frac{\left[n \left(n-1 \right)\left(n-2 \right)\cdots \left(n-\lfloor n \rfloor \right) \right]x^{n-\lceil n \rceil}}{e^x}=0</math> | ||
+ | ::This completes the proof. | ||
== Outside Links == | == Outside Links == | ||
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[[Category:Mathematics]] | [[Category:Mathematics]] | ||
+ | [[Category:Calculus]] |
Revision as of 04:32, January 26, 2010
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.