# Homework Help: Limit of a trigonometric function

1. Jul 15, 2016

### chwala

• Member warned that an attempt must be shown
1. The problem statement, all variables and given/known data
Mod note: Edited the following to fix the LaTeX

compute

$\lim_{n \rightarrow +0} \frac {8-9cos x+cos 3x} {sin^4(2x)}$

$\lim_{n \rightarrow +\infty} \frac {\sin(x)} x$

$\lim_{n \rightarrow +\infty} \frac {\sin(x)} x$

ok find limit as x→0 for the function $[ 8-9cos x + cos 3x/{sin^4}2x]$

2. Relevant equations

3. The attempt at a solution
use Lhopital rule...

Last edited by a moderator: Aug 17, 2016
2. Jul 15, 2016

### BvU

Well, 2 and 3 look alike and don't need L'H rule. You can deal with those yourself, right ? If not, what's the problem ?
1 and 4 are one and the same too ?

3. Jul 15, 2016

### chwala

sorry my interest is on 1 only...i was trying to post in latex form by using 2 and 3 as a hint.........

4. Jul 15, 2016

### BvU

No need to apologize, we just want to be clear on what we are looking at. So $$\lim_{n \downarrow 0} {\frac {8-9cos x+cos 3x} {sin^4}(2x)}$$ is the right one ?

(as you see, it's easy to come to the wrong expression...)

5. Jul 15, 2016

### chwala

ok find limit as x→0 for the function $[ 8-9cos x + cos 3x/{sin^4}2x]$
note that 2x is on the denominator...wish i could post this using latex...

6. Jul 15, 2016

### BvU

Now we are at $$\lim_{n \downarrow 0} 8-9\; cos x+ { cos 3x \over sin^4 (2x)}$$ which probably is not what your exercise intended.

7. Jul 15, 2016

### chwala

not clear post in latex please

8. Jul 15, 2016

### BvU

I though I did post using LaTex ? What is it that is not clear ?

9. Jul 15, 2016

### chwala

$sin^42x$ is the common denominator to all those terms in numerator and not the way you have posted. It is post number 4 which is correct with the only amendment on the 2x, that should be on the denominator as $sin^4.2x$

10. Jul 15, 2016

### Math_QED

Well you have posted it without the nessecary brackets so he couldn't know. He just copies what you wrote in the first post.

11. Jul 15, 2016

### BvU

Good. So we are at $$\lim_{n \downarrow 0} \frac {8-9cos x+cos 3x} {sin^4(2x)}$$
(the 2x in the wrong place was caused by me unintentionally ).

You mentioned l'Hopital. It is so to say your attempt at solution. What is it and what does it yield here ?

12. Jul 15, 2016

### Math_QED

I doubt that is the exercise though, since the limit does not depend on n, assuming that there is no correlation between n and x.

Most likely, it is: $\lim_{x \downarrow 0} \frac {8-9cos x+cos 3x} {sin^4(2x)}$

13. Jul 15, 2016

### Ray Vickson

It would be nice to post in LaTeX, but you can post clear and unambiguous questions simply by using parentheses properly! For example, here is your question in plain text, but using parentheses: (8 - 9 cos(x) + cos(3x))/sin^4(2x). Here is the same thing using in-line LaTeX: $(8 - 9 \cos x + \cos 3x)/\sin^4(2x)$, and again using "displayed" |aTeX:
$$\frac{8 - 9 \cos x + \cos 3x}{\sin^4 2x}$$
The first, plain-text version, is every bit as readable as the LaTeX versions, but of course it looks better. One advantage of the LaTeX versions is the need for fewer parentheses: $\cos 3x$ comes out perfectly clearly without the need for extra parentheses like $\cos (3x)$. (However, I felt is necessary to use parentheses for clarity in $\sin^4(2x)$ in the in-line version but not in the displayed version.) They are all necessary in the plain text version, but you can sometimes make that easier to read by using different parenthesis style, as in [8 - 9 cos(x) + cos(3x)]/sin^4(2x).

14. Jul 29, 2016

### chwala

Thanks sorry folks i have been on holiday...i will endeavour to look at this when i am free...........

15. Aug 17, 2016

### chwala

now using LHospital rule, we need to get the first derivative of
$\lim_{x \downarrow 0} \frac {8-9cos x+cos 3x} {sin^4(2x)}$ because substituting 0 will give us an indeterminate form...on applying the rule i am getting:,
$\lim_{x \downarrow 0} \frac {9sin x-sin 3x} {8cos 2x.sin^3(2x)}$ ok is this step correct?

16. Aug 17, 2016

### SammyS

Staff Emeritus
First of all, you are not taking the derivative of the overall expression. Rather you are taking the derivatives of the numerator and denominator respectively.

You also have a error in the derivative of the numerator.

17. Aug 18, 2016

### chwala

but hospital rule says we take the derivatives indepedently i.e of the numerator and the denominator indepedently.Why should i take the derivative of the whole expression?

18. Aug 18, 2016

### SammyS

Staff Emeritus
Right ! You should not take the derivative of the whole expression. But that's actually what you said you did in post #15 .

19. Aug 22, 2016

### chwala

If $f(x)= 8-9cos 2x + cos 3x ⇒ df/dx = 9 sin x - 3 sin 3x$
If $g(x)= {(sin 2x)^4} ⇒ dg/dx = 8cos 2x{(sin 2x)^3}$
and this is lhopital's rule what do you mean that i took derivative of whole expression?

20. Aug 22, 2016

### Math_QED

He meant that you said in post #15 that you wanted to take the derivative of whole expression.

21. Aug 22, 2016

### chwala

ok are my derivatives in post 19correct?, if so how do i move from there.........

22. Aug 22, 2016

### BvU

You take the limit for $x\downarrow 0$ and see what comes out.

23. Aug 22, 2016

### Math_QED

df/dx is wrong. Try to find the correct derivative of that function.

24. Aug 22, 2016

### chwala

$df/dx$? i dont understand..........

25. Aug 22, 2016

### BvU

f is short for numerator, like in: f/g.