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

In summary: In summary, the problem involves finding the derivative of y = cos(a^3 + x^3) using the chain rule. The attempted solution correctly identifies u = a^3 + x^3, but incorrectly uses the derivative of a constant, resulting in a final answer of -3sin(a^3 + x^3)(a^2 + x^2) instead of the correct answer of -3x^2sin(a^3 + x^3). The use of SIZE commands is not allowed on the forums.
  • #1
physics604
92
2

Homework Statement



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

Homework Equations



Chain rule

The Attempt at a Solution



y=cosu

[itex]\frac{dy}{du}[/itex]
= -sinu

u=a3+x3

[itex]\frac{du}{dx}[/itex]
= 3a2+3x2


[itex]\frac{dy}{dx}[/itex] = -3sin(a3+x3)(a2+x2).

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

Any help is much appreciated.
 
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  • #2
physics604 said:

Homework Statement



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

Homework Equations



Chain rule

The Attempt at a Solution



y=cosu

[itex]\frac{dy}{du}[/itex]
= -sinu

u=a3+x3

[itex]\frac{du}{dx}[/itex]
= 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.
 
  • #3
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.
 

1. What is the chain rule in calculus?

The chain rule is a formula in calculus used to find the derivative of composite functions. It states that the derivative of a composite function is equal to the derivative of the outer function evaluated at the inner function, multiplied by the derivative of the inner function.

2. How is the chain rule applied in derivatives?

The chain rule is applied in derivatives by first identifying the outer function and the inner function. Then, the derivative of the outer function is evaluated at the inner function, and finally, the derivative of the inner function is multiplied by this result to get the overall derivative of the composite function.

3. Can the chain rule be used for any type of composite function?

Yes, the chain rule can be used for any type of composite function, as long as the functions are differentiable. This means that the functions must have a defined derivative at every point in their domain.

4. Is the chain rule the only method for finding derivatives of composite functions?

No, there are other methods such as the product rule and quotient rule, but the chain rule is the most commonly used method for finding derivatives of composite functions.

5. How do I know when to use the chain rule in a calculus problem?

The chain rule should be used when the function you are trying to find the derivative of is a composition of two or more functions. It is also helpful to use the chain rule when the functions involved are more complicated and cannot be easily differentiated using other rules.

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