Homework Help: Find limit for rational function

1. Feb 20, 2017

late347

1. The problem statement, all variables and given/known dataf

f(x)= x^2 +4
find the limit as x approaches 1, there is something wrong with the latex code but I don't know what.
Limit $$\lim_{x\to 1} \frac{{f(x)}^4-{f(1)}^4}{x-1}$$
2. Relevant equations
-methods for finding limits
-factorising polynomials
-possibly polynomial long division (I used it)

3. The attempt at a solution

Well, I was a little bit bored today and decided to try to do some "more difficult" problems in my textbook for limits. You are free to comment on my polynomial long division if you want to. I tried to use it for this problem. Because for rational function if you can cancel out the pesky denominator.... then you can plug in the x= something and get the limit that way.

The value of the rational clause comes out at 0/0 just something to note in the beginning. Existence of limit as x->1 is still possible. We cannot immediately plug in x=1 and get the value of the limit of the rational function.

But, we ought to try to cancel the clause of function f so we are able to plug in x=1

If the expression is not cancellable, then we are going to have to start tabulating values for plus-side limit and minus-side limit, and evaluate if the both-sided limit exists.

I did the problem the tedious and long way, but it seemed to bring the correct answer eventually.

I was wondering if there's any easier way (faster way, smarter way) to get the factorization correct. Well... you could put the expression into wolfram alpha and have the computer factor it.

Foiling out the expression with a little bit pen-and-paper calculation

$[(x^4+8x^2+16)^2 -625]/(x-1)$

=
foiling out more with pen-and-paper
$(x^8+16x^6+96x^4+256x^2-369)/(x-1)$

putting that into polynomial long division in the Anglo-American format of the long division.

After some time, the remainder comes out at zero (which is good) because it implies our original expression was indeed factorable.

pic of polynomial long division. I attempted to follow the instruction in the Finnish language wiki article as best as I could. Anglo_american long division is indeed taught as the only long division in most schools I reckon...

The quotient is $(x^7+x^6+17x^5+17x^4+113x^3+113x^2+369x+369)$
and remainder =0

Hence we can continue and cancel the expression with (x-1) out of there

$\lim_{x\to 1}\frac{[x-1](x^7+x^6+17x^5+17x^4+113x^3+113x^2+369x+369)}{x-1}$

$\lim_{x\to 1}{(x^7+x^6+17x^5+17x^4+113x^3+113x^2+369x+369)}$

plug in x=1 and we will have the limit = 1000, as x->1

Last edited by a moderator: Feb 20, 2017
2. Feb 20, 2017

Staff: Mentor

Well, the result seems to be correct. But why haven't you defined $g(x)=(x^2+4)^4$ instead? If you write the limit with $g(x)$ instead of $f(x)$, does it look familiar to you?

3. Feb 20, 2017

late347

no I'm sort of re-learning this hopefully properly this time...

4. Feb 20, 2017

Staff: Mentor

Do you know, how the differential at a point is defined?

5. Feb 20, 2017

late347

it's in the next chapter in my textbook but yes, I have a basic idea how it would be done. https://simple.wikipedia.org/wiki/Difference_quotient

in our college last year, we did thing such as graphic derivation or whatever it was called. So I'm a little bit familiar with it. Maybe I will read more into the matter with my textbook today and tomorrow.

6. Feb 20, 2017

Staff: Mentor

7. Feb 20, 2017

Staff: Mentor

You had \lim_{x\to\1}. You shouldn't have \1 (i.e., with a backslash in front of 1), and you shouldn't cram everything in so tightly, with no spaces between things.

8. Feb 20, 2017

Ray Vickson

Since you already have the solution, I feel OK about offering a simpler solution that avoids long division and almost everything else. First a fairly standard trick: since your ratio $r(x)$ has $x^2$ in the numerator, it is convenient to try to get $x^2$ in the denominator; we can do that by multiplying and dividing by $x+1$, to get
$$r(x) = (x+1) \frac{(x^2+4)^4 - 5^4}{x^2-1}.$$
To make this even simpler, let $x^2-1 = z$, so
$$r(x) = (x+1) \frac{(z+5)^4 - 5^4}{z},$$
and we are looking at the $z \to 0$ limit. Since the numerator vanishes when $z = 0$, it will be a polynomial of degree 4 in $z$, and with no constant term; that is, $(5+z)^4 - 5^4 = c_4 z^4 + c_3 z^3 + c_2 z^2 + c_1 z = z(c_1 + c_2 z + c_3 z^2 + c_4 z^3).$ Therefore, the ratio is just $r(x) = (x+1) (c_1 + c_2 z + c_3 z^2 + c_4 z^3)$, whose limit as $x \to 1$ (and $z \to 0$) is just $2 c_1$. Here, $c_1$ is the coefficient of $z^1$ in the expansion of $(5+z)^4.$

Last edited: Feb 20, 2017
9. Feb 21, 2017

late347

Where do you get the whole business about $c_1,~c_2....$#

Can you get some of those from pascal's triangle. I got lost a little bit there at the end also I got lost a little bit about the end conclusion that limit should be 2*$c_1$

I think it could be easier to see whats going on if I saw it in limit notation... maybe I'll try to go thru the motions of that calculation with pen and paper first since Im writing on my phone now

10. Feb 21, 2017

Ray Vickson

The quantity $(z+5)^4-5^4$ is a polynomial of degree 4 in $z$, so the powers $z^1, z^2, z^3, z^4$ have some numerical coefficients---call them four different names, like $c_1, c_2, c_3, c_4$. They are easy enough to get directly if we do some more work: $(z+5)^4 - 5^4 = z^4+20 z^3+150 z^2+500 z$, but why bother to calculate them until we need them? It turns out that we do not need to know that $c_4 = 1, c_3 = 20, c_2 = 150$; in the end, all we need is $c_1 = 500$. Why is that enough? Well, what do YOU think is the answer to $\lim_{z \to 0} (c_4 z^3 + c_3 z^2 + c_2 z + c_1)?$

11. Feb 22, 2017

late347

well, for an expression like that, it looks like it is continuous, so you could just plug in 0 and get the value of the limit.
In this case $c_1$ is independant from z itself, so the $c_1$ remains the same. But the multiplications with z*c cancel out.
So the limit is $c_1$ in that case

But in the earlier example you actually had z in divisor, and the dividend both. So you said the z approaches zero, yes indeed it approches it... but the numerator z=0 and z the numerator vanishes??? I was a little worried there

12. Feb 22, 2017

Ray Vickson

No, I did not put $z = 0$ until after cancelling it. I never, ever, divided by 0. Basically,
$$\lim_{z \to 0} \frac{500z}{z} = \lim_{z \to 0} 500 = 500,$$
because as long as $z \neq 0$ we have $500 z / z = 500.$

13. Feb 22, 2017

ecastro

You might want to use this relationship to factor the numerator:

$\begin{equation*} x^2 - y^2 = \left(x + y\right) \cdot \left(x - y\right) \end{equation*}$

14. Feb 22, 2017

late347

I see that now... I had to walk through your solution with pen-and-paper. I used to think my English was rather good but sometimes it is hard to find the mathematical meaning from written text together with the math notation.

So at some point you actually cancelled the Zs from divisor and dividend?

Let's try some latex code here once again, I did the similar thing in pen-paper style, but let's try the the code and see if it come out ok

begin by expanding by (x-1), we are actually there already and we did the $(a+b)(a-b)=a^2-b^2$
$$\lim_{x\rightarrow 1} \frac{(x+1)*(x^2+4)^4-625}{x^2-1}$$

$( x^2 -1 ) = z$ plug it into the divisor and inside the ()^4 term

\begin{align} & \lim_{x \rightarrow 1;z \rightarrow 0}= (x+1)* \frac{(z+5)^4-5^4}{z} \nonumber \\ \leftrightarrow & \lim_{x \rightarrow 1; z \rightarrow 0}= (x+1) * \frac{z^4 +20z^3 +150z^2 +500z +625 -625}{z} \nonumber \\ \leftrightarrow & \lim_{x \rightarrow 1; z \rightarrow 0}= \frac{(x+1)*[z(z^3 +20z^2 +150z +500)]}{z} \nonumber \\ \end{align}

x^2-1=z
then one z variable can be cancelled from divisor and dividend

$\leftrightarrow \lim_{x \rightarrow 1}= [x+1]*[(x^2-1)^3 +20(x^2-1)^2 +150(x^2-1) +500] \nonumber \\$
looks like the limit will be sometihng like

2*(500) because now the entire polynomial thingy is continuous, because the divisor was cancellable and cancelled out. We just plug in the x->1 x=1 and find the value of the limit.

15. Feb 22, 2017

Ray Vickson

Even shorter: use $a^4 - b^4 = (a-b)(a^3 + a^2 b + a b^2 + b^3),$ so
$$\begin{array}{rcl}(x^2+4)^4-5^4 &=& (x^2+4-5)((x^2+4)^3 +5 (x^2+4)^2 + 5^2 (x^2+4) + 5^3) \\ &=& (x-1)(x+1) ((x^2+4)^3 +5 (x^2+4)^2 + 5^2 (x^2+4) + 5^3) \end{array}$$
We can cancel out the $(x-1)$ factor, then put $x=1$ in the rest of the terms to get the limiting ratio.