# Derivatives: help?

1. Aug 19, 2008

### Wholewheat458

1. The problem statement, all variables and given/known data
Prove using the definition of a derivative.
If y=sinx, then y1=cosx

2. Relevant equations
.. ? identities maybe..? i dont really know much about derivatives..

3. The attempt at a solution
y = sinx..
y1 = (1)sinx^1-1
y1 = 1..
how is that suppose to equal cosx?? !
i havent got a clue, any help would really be appreciated!!
:shy:

2. Aug 19, 2008

### cristo

Staff Emeritus
What is the definition of the derivative?

3. Aug 19, 2008

### Wholewheat458

?? i'm not all that certain..
its got somethink to do with graphs, but..
idk, i been looking at a graph of y = sinx
but i dont know what to do with it: oy

4. Aug 19, 2008

### cristo

Staff Emeritus
5. Aug 19, 2008

### Wholewheat458

.. ?
woahw.. so... how do i apply that
to the sin graph?..
: no clue..
maybe i'll go over it again..

6. Aug 19, 2008

### cristo

Staff Emeritus
You set f(x)=sin(x) in the expression $$f'(x)=\lim_{h\to0}\;\frac{f(x+h)-f(x)}{h}$$

Hint: if you actually read through that thread, then you may be surprised as to what you find...

I'm moving this to calc&beyond, since it is calculus!

7. Aug 19, 2008

### Wholewheat458

.. i dont think i understand the formula..
: f(x) = sinx .. then.
= sinx = lim(h-->) sin(x+h) - sinx/ h
... this is so wrong
wait, what is h?..

8. Aug 19, 2008

### cristo

Staff Emeritus
Have you studied limits before? From your posts here, it sounds like you haven't. This problem will be incredibly difficult if you have not heard about limits before, and I can't see why it would have been set for you to try.

9. Aug 19, 2008

### Wholewheat458

^^ i dont know why either!!
ahhahaa!! i think i'll just skip it for now..

10. Aug 19, 2008

### Feldoh

Try learning limits, then the definition of derivative, then the squeeze theorem and then you should be able to make a fairly decent proof of it.

11. Aug 19, 2008

### rootX

Skipping is not possible if you are learning about derivatives right now.
This will haunt you later