# Math Help - Quick Question.

1. ## Quick Question.

What's happening in this derivative?

$-5e^{xcos(x)}$

Ends as:

$5*x*e^{x*cos(x)}*sin(x)-5*cos(x)*e^{x*cos(x)}$

From what I know.

$-5e^u*u'$

Does not get me to that end result, where u equals (x*cos(x)). Is there a special rule in ending with derivatives like this?

2. Originally Posted by Zanderist
$-5e^u*u'$

Does not get me to that end result, where u equals (x*cos(x)). Is there a special rule in ending with derivatives like this?
This formula is correct ..
In your problem $u=x \, cos(x)$ ..
Apply the formula .. and notice that you will need the product rule to find $u'$ ..

3. oh right.

4. BTW, you should put brackets like this:

Originally Posted by Zanderist
$5xe^{xcos(x)} {\color{red} ( } sin(x)-5cos(x)e^{xcos(x)} {\color{red} ) }$