Partial Derivative - Chain Rule

**dbakeg00** · September 1st 2011, 06:17 AM

I have this problem which I am stuck on. It tells me to find the derivative of the function in 2 ways...first by substitution then by the chain rule using partial derivatives. I found the first one fine using substitution then taking a derivative. The second one I came up with a different answer (which I believe is incorrect). Can someone help me get unstuck please? I've attached a picture of my work. Thanks

**Siron** · September 1st 2011, 06:26 AM

I didn't check whole the answer but I notice a mistake by calculating $\frac{dz}{dy}$ with the chain rule:
$\frac{d}{dy}\left(\frac{x}{x^2+y^2}\right)=x\cdot \frac{d}{dy}(x^2+y^2)^{-1}$
$=-x\cdot(x^2+y^2)^{-2}\cdot 2y =\frac{-2xy}{(x^2+y^2)^2}$

Which is different from your answer.

**dbakeg00** · September 1st 2011, 06:41 AM

That was it. Dumb error. Thanks for the help!

**Siron** · September 1st 2011, 06:41 AM

You're welcome!

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