Derivative of y=cos(a3+x3)What is the derivative of y=cos(a3+x3)?

[physics604]

Homework Statement

Find the derivative of y=cos(a³+x³)

Homework Equations

Chain rule

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.

 

[LCKurtz]

Homework Statement

Find the derivative of y=cos(a³+x³)

Homework Equations

Chain rule

The Attempt at a Solution

y=cosu

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

u=a³+x³

$\frac{du}{dx}$ = 3a²+3x²

$a$ 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.

 

[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.

 

[LCKurtz]

I don't know how to make the equations like yours.

It's easy. Look here (from the pinned FAQ at the top of this forum):

https://www.physicsforums.com/showpost.php?p=3977517&postcount=3