# Calculate limits

1. Oct 1, 2013

### lep11

1. The problem statement, all variables and given/known data
1. Calculate lim x->0 (xtanx/cos(2x)-1) without using L'Hospitals rule.

2. Relevant equations
I am told that lim x->0 (sinx/x)=1

3. The attempt at a solution
If I substitute 0 in it gets 0/0. I have tried several trig identities without luck.

2. Oct 1, 2013

### arildno

Formulate the double-angle identity, and in particular, replace cos(2x)-1 with a suitable expression with a trig function with "x" in its argument, rather than "2x"

3. Oct 1, 2013

4. Oct 1, 2013

### ppham27

I'd also note that
$$\lim_{x \rightarrow p} \frac{f(x)}{g(x)} = \frac{\lim_{x \rightarrow p} f(x)}{\lim_{x \rightarrow p} g(x)}.$$

5. Oct 1, 2013

### Saitama

Correct. Write tan in terms of sin and cos.

6. Oct 1, 2013

### arildno

An excellent choice of identity!

Use that in your example, and simplify your expression. Remember that 2cos(x)sin(x)=sin(2x) will be very handy indeed for further simplifications. (It is not strictly necessary to invoke)

Last edited: Oct 1, 2013
7. Oct 1, 2013

### arildno

In general, completely incorrect.

Try it on lim_infty x/x, for example.

8. Oct 1, 2013

### ppham27

Sorry, I should clarify that it's true provided that limits of $f$ and $g$ exist at $p$ and the limit of $g$ at $p$ is nonzero.

9. Oct 1, 2013

### arildno

Then we are in agreement.

10. Oct 1, 2013

### lep11

hmm...xtanx/(cos(2x)-1)=xsinx/cosx/-2(sinx)2=xsinx/-2cosx(sinx)2=xsinx/-sinxsin(2x)

11. Oct 1, 2013

### arildno

=-x/sin(2x)=-1/2*(2x/sin(2x))

Can you finish it off from here?

12. Oct 1, 2013

### Saitama

Why simplify to sin(2x)? Keep it simple.

Use the relevant equation you have posted.

13. Oct 1, 2013

### arildno

I put him on that path; I believe my personal sense of aesthetics interfered with what is "simplest"

If OP does not use my hint there, he'll arrive at -1/(2cos(x))*(x/(sin(x)) which I personally find ugly, but which might, possibly, be regarded as objectively simpler.

14. Oct 1, 2013

### lep11

Not sure It's still giving me trouble.

Last edited: Oct 1, 2013
15. Oct 1, 2013

### arildno

Well, if x goes to zero, then y=2x also goes to zero, doesn't it?
So, you could evaluate your limit as calculating:
-1/2*lim_y->0(y/sin(y))

16. Oct 1, 2013

### lep11

lim x->0 (sinx/x)=1 and lim x->0 (x/sinx)=1???

17. Oct 1, 2013

### arildno

That's right!
So, lim y->0 y/sin(y)=??

18. Oct 1, 2013

### lep11

=1... I was just unsure if lim y->0 y/sin(y)=1 thanks a lot

19. Oct 1, 2013

### lep11

=1 but why?

20. Oct 1, 2013

### arildno

Note that with limits, the symbol used is NOT the important thing; rather, it is how that symbol (either "y" or "x" in this case) appears distributed in the formula that is important for the evaluation of the limit.

"2x" (i,e, "y") plays the same role in the limiting process as "x" does in your received formula.
---------------
Is there STILL a difference?
Sure, if we are nitpicky, and require that the "x" is in "2x" is the SAME "x" as in x/sin(x).

For each choice of "x", "2x" will have double the value of "x". Thus, if you look at a sequence of x's converging to 0, precisely the same sequence with "x" replaced by "2x" will, typically, be SLOWER in going towards zero than the single x will. (take the sequence of x's like 1/n, the 2x's go as 2/n, both go to 0 as n goes to infinity, but at different rates)

But, both the "x" and the "2x" will reach the same point in the end. It has no bearing on the actual limit value (THE end point of the limiting process!!), but a certain bearing on the "time" the limiting process takes.