Question

find the derivative of the function cosx

Answer 1

-sinx

Answer 2

if you remember cos always goes to -sin when you differentiate
then think of x as a functon
so you have
f(x)=x
differentiate you get
f'(x)=1
1 goes infront of sin

you get -sinx

Answer 3

k i understand thnx. but is there any way to solve it using trig formula

Answer 4

you mean starting with the difference quotient?

Answer 5

I have to go
the derivation is long, and relies on knowing \[\lim_{x\to0}\frac{\sin x}x\text{ and }\lim_{x\to0}\frac{\cos x-1}x\]and sum of angles trig identity

Answer 6

here is a link for the proof of sinx
the one for cos is similar enough that you should be able to use the same ideas

Answer 7

sorry :p
http://oregonstate.edu/instruct/mth251/cq/Stage6/Lesson/sine.html