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# Product and Quotient Rule!!! watch

1. There's a question which confuses me because of its wording.

Given that u=(1+x)/(1-x) , find du/dx.

I can do that part, but then the following wording confuses me:

using this result, and the chain rule, show that if y=((1+x)/(1-x))^3 , dy/dx = (6(1+x)^2)/((1-x)^4).

I can do the second part using the quotient rule. Does that answer the question?

Thanks
2. You have to use the chain rule for the second part since it is a function within a function.

dy/dx = du/dx x dy/du

3. I thought about that, but then where does dy/du come from?
4. (Original post by thegold94)
I thought about that, but then where does dy/du come from?
Well, (x+1)/(x-1) is u.
So you can substitute this into the second equation they give you and you will see what I mean.
5. I think I've got it. If u= (1+x)/(1-x), then I think I take y=u^3, differentiate that to dy/du = 3u^2, stick u in there and then end up multiplying 3(1+x)^2 / (1-x)^2 to get 6(1+x)^2 / (1-x)^4. That actually works. Thanks
6. (Original post by thegold94)
I think I've got it. If u= (1+x)/(1-x), then I think I take y=u^3, differentiate that to dy/du = 3u^2, stick u in there and then end up multiplying 3(1+x)^2 / (1-x)^2 to get 6(1+x)^2 / (1-x)^4. That actually works. Thanks
You're welcome.

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Updated: November 27, 2011
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