# Thread: Finding the value of a derivative

1. ## Finding the value of a derivative

Find the value of the derivative of the function at (╥, -1)
Function: g(t) = 2 + 3 cos t

My work:
Plugged in ╥ to t
2 + 3 cos (╥)

Then I turn it into g' (g prime)
The rule is usually multiply the exponent into the coefficient than minus one from the exponent.
For example y = 4x^2 + 4x + 1
y' = 8x + 4

Can anyone help me with 2 + 3 cos t.

Thanks!

2. ## Re: Finding the value of a derivative

You want to differentiate first, then evaluate the derivative at $t=\pi$.

Do you know how to differentiate the cosine function?

3. ## Re: Finding the value of a derivative

Originally Posted by MarkFL2
You want to differentiate first, then evaluate the derivative at $t=\pi$.

Do you know how to differentiate the cosine function?
Oh! Doesn't cos becomes -sin then.

4. ## Re: Finding the value of a derivative

Yes, so you have:

$g'(t)=-3\sin(t)$

Now, you may evaluate the derivative for $t=\pi$.