How to prove there is no limit of cos1/x using theorm of limit

1. Oct 12, 2004

flying2000

How to prove there is no limit(x->0) of cos1/x using theorm of limit?
Anybody can give me some hints?
thanks

Last edited: Oct 12, 2004
2. Oct 12, 2004

Dr-NiKoN

I'm just learning this myself, but here is how I would approach it.

$\lim_{x\to 0}f(x)=\cos(\frac{1}{x})$
You know what happens to $\lim_{x\to 0}f(x) = \frac{1}{x}$

You also know what a graf with $f(x) = \cos(x)$ looks like.

Now consider

$y = \frac{1}{x}$

and

$\lim_{y\to\infty}f(y)=\cos(y)$

Last edited: Oct 12, 2004
3. Oct 12, 2004

flying2000

thanx

Choose an arbitrary number called "L". (Think of it as between -1 and 1 if you like)
Show that there exists a neighbourhood U of 0, so that given |L|<1, there will always be some x in U so that |L-cos(1/x)|>1/2
but I don't know how to continue....

4. Oct 12, 2004

flying2000

my question is how to find x to makes |L-cos(1/x)|>1/2

my question is how to find x to makes |L-cos(1/x)|>1/2

5. Oct 13, 2004

matt grime

Use the easier, and equivalent, definition of limit.

If you can find two sequences a(n) and b(n) tending to zero such that cos(a(n)) and cos(b(n)) tend to different numbers you're done.

6. Oct 13, 2004

flying2000

thanx,but I want to know if I can prove it by E-Delta therom? Hope U can Help me..

thanx,but I want to know if I can prove it by E-Delta therom? Hope U can Help me..

7. Oct 13, 2004

jebeagles

Take limit as x approaches 0 from the right (lim->0+) of cos(1/x). Using direct substitution, you would get cos(infinity). cos(infinity) does not approach a single value, because it is not a monotonic function over the required interval. So, no limit exists, because no single value is approached. If one side of a limit does not exist, the limit does not exist.

This is the way I learned to do limits. Not sure if it applies here though.

8. Oct 13, 2004

matt grime

To prove the counter example it suffices to show that given any d there is a e such that there is some x with |cos(x)-L| > e and |x|<d

but this is trivial. firstly L must be between -1 and 1, and let x be some sufficiently large solution to cos(x)=1 or -1, then one of |1-L| and |-1-L| must be greater than 1 (which we can choose to be e)

nb e:=epsilon, d:=delta

9. Oct 13, 2004

flying2000

thanx

I got it now,thanx a lot
Actually,I thought it should hold for all x ,my mistake!

10. Oct 14, 2004

matt grime

In the negation of propositions never forget that 'for all' is changed to 'there exists'