# Help me solve f(x)=7arctan(2sin(3x)), f′(x)=?

## Based on what I did it should be [42cos3x]/[3-2cos(6x)] but that's wrong.

##### 1 Answer
Feb 17, 2018

(42cos(3x))/(1+4(sin^2(3x))

#### Explanation:

You'll have to memorize this rule:
$\frac{d}{\mathrm{dx}} \left[\arctan \left(u\right)\right] = \frac{u '}{1 + {u}^{2}}$
Use chain rule to find:
u'=$6 \cos \left(3 x\right)$
Plug that and into the rule and remember to multiply by that pesky seven!