In calculus, it is usually possible to compute the limit of the sum, difference, product, quotient or power of two functions by taking the corresponding combination of the separate limits of each respective function. For example,
and likewise for other arithmetic operations; this is sometimes called the algebraic limit theorem. However, certain combinations of particular limiting values cannot be computed in this way, and knowing the limit of each function separately does not suffice to determine the limit of the combination. In these particular situations, the limit is said to take an indeterminate form, described by one of the informal expressions
where each expression stands for the limit of a function constructed by an arithmetical combination of two functions whose limits respectively tend to or as indicated.
A limit taking one of these indeterminate forms might tend to zero, might tend to any finite value, might tend to infinity, or might diverge, depending on the specific functions involved. A limit which unambiguously tends to infinity, for instance is not considered indeterminate.[1] The term was originally introduced by Cauchy's student Moigno in the middle of the 19th century.
The most common example of an indeterminate form is the quotient of two functions each of which converges to zero. This indeterminate form is denoted by
0/0
x
0,
x/x3
x/x
x2/x
infty
1
0
0/0
0/0
0
1
infty
x\sin(1/x)/x
f(x)
g(x)
0
x
c
An expression that arises by ways other than applying the algebraic limit theorem may have the same form of an indeterminate form. However it is not appropriate to call an expression "indeterminate form" if the expression is made outside the context of determining limits.An example is the expression
00
1
0infty
The indeterminate form
0/0
As mentioned above,whileThis is enough to show that
0/0
x
0/0
a
infty
See main article: Zero to the power of zero. The following limits illustrate that the expression
00
Thus, in general, knowing that
style\limxf(x) = 0
style\limxg(x) = 0
If the functions
f
g
c
f
x
c
f(x)g(x)
1
The expression
1/0
f/g
f
1
g
0,
f
g
f/g
+infty
f/g
-infty
In each case the absolute value
|f/g|
+infty
f/g
infty
a/0
a\ne0
a=+infty
a=-infty
The expression
0infty
0+infty
\limxf(x)g(x)
0,
f(x)
x
c
0-infty
1/0
f(x)>0
x
c
+infty
To see why, let
L=\limxf(x)g(x),
\limx{f(x)}=0,
\limx{g(x)}=infty.
\limxln{f(x)}=-infty,
lnL=\limx({g(x)} x ln{f(x)})=infty x {-infty}=-infty,
L={e}-infty=0.
The adjective indeterminate does not imply that the limit does not exist, as many of the examples above show. In many cases, algebraic elimination, L'Hôpital's rule, or other methods can be used to manipulate the expression so that the limit can be evaluated.
When two variables
\alpha
\beta
style\lim
\beta | |
\alpha |
=1
\alpha\sim\beta
Moreover, if variables
\alpha'
\beta'
\alpha\sim\alpha'
\beta\sim\beta'
Here is a brief proof:
Suppose there are two equivalent infinitesimals
\alpha\sim\alpha'
\beta\sim\beta'
For the evaluation of the indeterminate form
0/0
x\sim\sinx
For example:
In the 2nd equality,
ey-1\simy
y=xln{2+\cosx\over3}
y\simln{(1+y)}
y={{\cosx-1}\over3}
1-\cosx\sim{x2\over2}
See main article: L'Hôpital's rule. L'Hôpital's rule is a general method for evaluating the indeterminate forms
0/0
infty/infty
f'
g'
f
g
infty/0
1/0
L'Hôpital's rule can also be applied to other indeterminate forms, using first an appropriate algebraic transformation. For example, to evaluate the form 00:The right-hand side is of the form
infty/infty
f
g
f
Although L'Hôpital's rule applies to both
0/0
infty/infty
f/g
(1/g)/(1/f)
The following table lists the most common indeterminate forms and the transformations for applying l'Hôpital's rule.
Indeterminate form | Conditions | Transformation to 0/0 | Transformation to infty/infty | |||||||
---|---|---|---|---|---|---|---|---|---|---|
\limxf(x)=0, \limxg(x)=0 | \limx
=\limx
| |||||||||
\limxf(x)=infty, \limxg(x)=infty | \limx
=\limx
| |||||||||
0 ⋅ infty | \limxf(x)=0, \limxg(x)=infty | \limxf(x)g(x)=\limx
| \limxf(x)g(x)=\limx
| |||||||
infty-infty | \limxf(x)=infty, \limxg(x)=infty | \limx(f(x)-g(x))=\limx
| \limx(f(x)-g(x))=ln\limx
| |||||||
00 | \limxf(x)=0+,\limxg(x)=0 | \limxf(x)g(x)=\exp\limx
| \limxf(x)g(x)=\exp\limx
| |||||||
1infty | \limxf(x)=1, \limxg(x)=infty | \limxf(x)g(x)=\exp\limx
| \limxf(x)g(x)=\exp\limx
| |||||||
infty0 | \limxf(x)=infty, \limxg(x)=0 | \limxf(x)g(x)=\exp\limx
| \limxf(x)g(x)=\exp\limx
|