using def'n of derivative, find f^'(x) if f(x)= cosx

Question

using def'n of derivative, find  f^'(x) if f(x)= cosx

isaiah.feynman · Answer

The definition of the derivative is $$\lim_{h \rightarrow 0}\frac{ f(x+h)-f(x) }{ h } $$

anonymous · Answer

yes, I know that. 
I get as far as 
$$\lim_{h \rightarrow 0}\frac{ \cos(x+h)-cosx) }{ h }$$

and then i am lost..

zarkon · Answer

http://mathworld.wolfram.com/TrigonometricAdditionFormulas.html

isaiah.feynman · Answer

Yes. You need to evaluate cos(x+h) using an identity.

anonymous · Answer

i was not given any identities…. I obviously know a sin has to come from somewhere in order to get and answer of -sinx

anonymous · Answer

thanks for identities. I have it figured out now… strange how a prof can ask a question on an assignment and not provide info to answer…. anyways, THANKS AGAIN!!

anonymous · Answer

Hey can you show me how you solved it? Im stuck

isaiah.feynman · Answer

I think your prof expects you to have learned your algebra well. No clues for assignments

anonymous · Answer

@Isaiah.Feynman Can you show me the steps cuz Im getting stuck

anonymous · Answer

i am taking a photo of my notes now. one second

anonymous · Answer

ohh great thanks :D

anonymous · Answer

attachment

anonymous · Answer

tried a billion things to send it but wouldn't save so I sent a screen shot of photo. hopefully you can see it

anonymous · Answer

Neat handwriting :) Ya must be a female

anonymous · Answer

Ohhhh I seeee . Thanks you

anonymous · Answer

haha yes i am female. and no problem!