# Thread: derivative of trig function

1. ## derivative of trig function

what is the derivative of cos^2x ?

i know cosx = -sinx

and i dont think cos^2x would equal -sin^2x because i would probably more complicated then that... lol

am i right or am i wrong... i need some guidance please. thank you.

2. Write cos^2 x as (cos x)^2 and then use the chain rule.

3. okay so this is how i did it
cosx^2

y= u^2
dy/du = 2u
u= cosx
dy/dx = -sinx
x=

dy/dx = dy/du X du/dx
= 2u X (-sinx)
= -2cosxsinx
= -sin2x ? <cause i use that identity because it is sin2x = 2sinxcosx...

4. is that right?

5. Perfect. Well done. Remember that little trick for next time...and there will be a next time!

6. yeah forsure thanks, i would have never thought to do that myself. =)