# Can I get help finding the derivative?

• Aug 5th 2009, 08:18 AM
Darkhrse99
Can I get help finding the derivative?
$\displaystyle t^3cost-13tsint-13cost$

Thanks
Jason
• Aug 5th 2009, 08:25 AM
AlephZero
Quote:

Originally Posted by Darkhrse99
$\displaystyle t^3cost-13tsint-13cost$

Thanks
Jason

The derivative is found using relatively straight-forward methods (Product Rule and Summing Rule). Can you show your work, so that we can see what you've done?
• Aug 5th 2009, 09:46 AM
Darkhrse99
I came up with $\displaystyle -3t^2sint-13cost+13sint$

Is this correct?
• Aug 5th 2009, 09:58 AM
Failure
Quote:

Originally Posted by Darkhrse99
I came up with $\displaystyle -3t^2sint-13cost+13sint$

Is this correct?

Not quite. You do not seem to have applied the product rule correctly.

I get the following (for the the first two terms you need to apply the product rule)

$\displaystyle \begin{array}{l} (t^3\cos t-13t\sin t-13\cos t)'\\ \quad =3t^2\cdot\cos t+t^3\cdot(-\sin t)-13\cdot \sin t-13t\cdot\cos t-13(-\sin t)\\ \quad = 3t^2\cos t -t^3\sin t-13t\cos t \end{array}$