# Thread: 2nd derrivative of y=coskt

1. ## 2nd derrivative of y=coskt

Am i doing this right?

y' = -sinkt
y'' = -coskt

??

thanks.

2. Originally Posted by johntuan
Am i doing this right?

y' = -sinkt
y'' = -coskt

??

thanks.
Close...you forgot to apply the chain rule!!

Do these again, and then check the spoiler to see if you're right.

Spoiler:

$y^{\prime}=-k\sin kt$

$y^{\prime\prime}=-k^2\cos kt$

3. ahhh...I can't believe I forgot to do such a function..