Infinity is a really special idea. We recognize we can"t reach it, yet we deserve to still try to work-related out the worth of functions that have actually infinity in them.

You are watching: Limit of (-1)^n

One separated By Infinity

Let"s begin with an amazing example.

Why don"t us know?

The simplest factor is the Infinity is no a number, the is an idea.

So 1 is a little bit like saying 1beauty or 1tall.

Maybe we might say that 1= 0, ... Yet that is a difficulty too, since if we division 1 into unlimited pieces and they end up 0 each, what happened to the 1?

In truth 1 is well-known to be undefined.

But us Can method It!

So instead of do the efforts to work it the end for infinity (because we can"t get a judicious answer), let"s shot larger and also larger values of x:



Now we can see that as x gets larger, 1x often tends towards 0

We room now challenged with an exciting situation:

We can"t to speak what happens as soon as x it s okay to infinityBut we deserve to see the 1x is going in the direction of 0

We desire to give the price "0" yet can"t, so instead mathematicians say specifically what is walk on by utilizing the distinct word "limit"

The limit of 1x together x ideologies Infinity is 0

And write it favor this:

limx→∞ (1x) = 0

In various other words:

As x viewpoints infinity, then 1x viewpoints 0

When you watch "limit", think "approaching"

It is a mathematical way of saying "we are not talking around when x=∞, however we understand as x gets bigger, the answer it s okay closer and also closer come 0".


So, sometimes Infinity can not be provided directly, but we can use a limit.

What wake up at ∞ is undefined ...1
... Yet we do understand that 1/x philosophies 0as x viewpoints infinity
limx→∞ (1x) = 0

Limits approaching Infinity

What is the border of this role as x viewpoints infinity?

y = 2x

Obviously together "x" gets larger, therefore does "2x":


So as "x" viewpoints infinity, climate "2x" likewise approaches infinity. We compose this:

limx→∞ 2x = ∞

yet don"t it is in fooled by the "=". We cannot in reality get come infinity, however in "limit" language the limit is infinity (which is really saying the role is limitless).

Infinity and also Degree

We have actually seen 2 examples, one visited 0, the various other went come infinity.

In fact many infinite limits are actually rather easy to job-related out, as soon as we number out "which means it is going", like this:

Functions choose 1/x method 0 together x approaches infinity. This is likewise true for 1/x2 etc

A function such as x will strategy infinity, and also 2x, or x/9 and also so on. Likewise functions v x2 or x3 etc will likewise approach infinity.

But it is in careful, a role like "−x" will approach "−infinity", for this reason we need to look at the indications of x.

Example: 2x2−5x

2x2 will certainly head towards +infinity−5x will head in the direction of -infinityBut x2 grows an ext rapidly 보다 x, therefore 2x2−5x will head in the direction of +infinity

In fact, when we look at the degree of the function (the highest possible exponent in the function) we can tell what is going to happen:

When the level of the duty is:

greater 보다 0, the border is infinity (or −infinity)less 보다 0, the limit is 0

But if the Degree is 0 or unknown then we should work a bit harder to discover a limit.

Rational Functions

A Rational role is one that is the ratio of two polynomials:
f(x) = P(x)Q(x)
For example, here P(x) = x3 + 2x − 1, and Q(x) = 6x2:
x3 + 2x − 16x2

Following on from ours idea that the level of the Equation, the very first step to uncover the border is come ...

Compare the Degree that P(x) come the Degree the Q(x):

If the level of p is much less than the level of Q ...

... The border is 0.

If the degree of P and Q room the same ...

... Division the coefficients of the terms through the largest exponent, choose this:


(note that the largest exponents are equal, together the level is equal)

If the level of ns is higher than the degree of Q ...

... Then the border is positive infinity ...

... Or maybe an adverse infinity. We have to look in ~ the signs!

We can work the end the authorize (positive or negative) by looking at the indications of the terms through the largest exponent, similar to how we found the coefficients above:

x3 + 2x − 16x2

For instance this will go to positive infinity, because both ...

x3 (the term with the biggest exponent in the top) and6x2 (the term through the biggest exponent in the bottom) ... Room positive.

−2x2 + x5x − 3
But this will certainly head for an adverse infinity, because −2/5 is negative.

A more tough Example: functioning Out "e"

This formula gets closer come the value of e (Euler"s number) together n increases:

(1 + 1n)n

At infinity:

(1 + 1 )∞ = ???

We don"t know!

So instead of trying to job-related it out for infinity (because we can"t gain a sensible answer), let"s try larger and larger values of n:


n(1 + 1/n)n

Yes, the is heading towards the worth 2.71828... i beg your pardon is e (Euler"s Number)

So again we have actually an weird situation:

We don"t understand what the value is when n=infinityBut we can see the it settles towards 2.71828...

So we use limits to write the answer prefer this:

limn→∞ (1 + 1n)n = e

It is a mathematical method of speak "we room not talking around when n=∞, however we understand as n gets bigger, the answer it s okay closer and closer come the worth of e".

Don"t do It The Wrong method ... !

If we try to use infinity as a "very big real number" (it isn"t!) we get:

(1 + 1)∞ = (1+0)∞ = 1∞ = 1 (Wrong!)

So don"t try using Infinity together a actual number: friend can obtain wrong answers!

Limits are the right way to go.

Evaluating Limits

I have actually taken a gentle approach to boundaries so far, and shown tables and graphs to illustrate the points.

See more: Which Of The Following Statements About Exchanges Is Not True? ?

But to "evaluate" (in various other words calculate) the value of a limit deserve to take a bit more effort. Discover out more at examining Limits.

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