## Some Special Limits

Here we will discuss some important limits that everyone should be aware
of. They are very useful in many branches of science.

**Example:** Show using the Logarithmic function that

,

for any *a* > 0.

**Answer:** Set . We have

. ln(a)

Clearly, we have

.

Hence,

which translates into

.

**Example:** Show that

.

**Answer:** We will make use of the integral while the **Hôpital
Rule** would have done a cleaner job. We have

so

.

For , we have , which is equivalent to
. Hence,

.

But,

.

Therefore, putting the stuff together, we arrive at

.

Since,

,

as *n* goes to and ,
the Pinching Theorem gives

.

The difficulty in this example was that both the numerator and
denominator grow when *n* gets large. But, what this conclusion shows
is that *n* grows more powerfully than .

As a direct application of the above limit, we get the next one:

**Example:** Show that

.

**Answer:** Set . We have

.

Clearly, we have (from above)

.

Hence,

,

which translates into

.

The next limit is extremely important and I urge the reader to be aware
of it all the time.

**Example:** Show that

,

for any number *a*.

**Answer:** There are many ways to see this. We will choose one
that involves a calculus technique. Let us note that it is equivalent
to show that

.

Do not worry about the domain of , since for large *n*, the
expression will be a positive number
(close to 1). Consider the function

and *f*(0) = 1. Using the definition of the derivative of ,
we see that *f*(*x*) is continuous at 0, that is, . Hence, for any sequence which
converges to 0, we have

.

Now, set

.

Clearly we have . Therefore, we have

.

But, we have

,

which clearly implies

.

Since

,

we get

.

The next example, is interesting because it deals with the new notion of
series.

**Example:** Show that

**Answer:** There are many ways to handle this sequence. Let us
use calculus techniques again. Consider the function

.

We have

and

,

for any . Note that for any , we have

,

hence

,

which gives

.

Since

,

we get

.

In particular, we have

.

Therefore, since , we must have

.

**
**

**
[Trigonometry]
[Calculus]
**** **
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[Algebra]
[Differential Equations]
[Calculus]
[Complex Variables]
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*Mohamed A. Khamsi *

Tue Dec 3 17:39:00 MST 1996

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