# Derivatives, chain rule 3

1. Nov 9, 2013

### physics604

1. The problem statement, all variables and given/known data

Find the derivative of y=cos(a3+x3)

2. Relevant equations

Chain rule

3. The attempt at a solution

y=cosu

$\frac{dy}{du}$
= -sinu

u=a3+x3

$\frac{du}{dx}$
= 3a2+3x2

$\frac{dy}{dx}$ = -3sin(a3+x3)(a2+x2).

The answer is supposed to be -3x2sin(a3+x3). What did I do wrong?

Any help is much appreciated.

2. Nov 9, 2013

### LCKurtz

$a$ is constant so the derivative of $a^3$ is $0$.

Please do not insert SIZE commands in your posts. It makes them very hard to follow when quoting them, aside from violating forum policy.

3. Nov 9, 2013

### physics604

Okay, thanks!

I was thinking maybe that was my error, but I wasn't 100% sure.

And I didn't know about the SIZE commands rule, so I'll stay away from them from now on. I just thought they'd be easier to read since I don't know how to make the equations like yours.

4. Nov 9, 2013

### LCKurtz

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