# Math Help - Chain Rule Problem

1. ## Chain Rule Problem

The question gives a reeeallly weird answer. Perhaps someone could confirm or help me out on this. It's extra practice for me.

Let y= u^3 + 6u , where u= 2x^4 + 3x^2 - 2 Find dy/dx. Leave your answer in terms of u and x.

2. Originally Posted by mike_302
The question gives a reeeallly weird answer. Perhaps someone could confirm....

3. well, what I did was subbed in the quartic function for 'u' and then applied the chain rule to the first part, and I expanded the second part out, and took its derivative. Now that gets me something messy. The answer in the back gives me some silly answer that has a 'u' and an 'x' in it. (3u^2+6)(8x^3+6x)

Makes no sense to me.

4. Originally Posted by mike_302
well, what I did was subbed in the quartic function for 'u' and then applied the chain rule to the first part, and I expanded the second part out, and took its derivative. Now that gets me something messy. The answer in the back gives me some silly answer that has a 'u' and an 'x' in it. (3u^2+6)(8x^3+6x)

Makes no sense to me.
It's fine.

If you wanted to, you could replace the u's with $2x^4 + 3x^2 - 2$, but it's not necessary... infact you've been told to keep it in terms of u as well as x.