I'm trying to follow a derivation in given in a text book. Part of this derivation goes like this:(adsbygoogle = window.adsbygoogle || []).push({});

[tex]\frac{d}{ds}\left(\frac{1}{c}\frac{dx}{ds}\right)=c\left(\frac{\partial^2\tau}{\partial x^2}\frac{\partial \tau}{\partial x} + \frac{\partial^2\tau}{\partial x \partial y}\frac{\partial \tau}{\partial y}\right)[/tex]

[tex]=\frac{c}{2}\frac{\partial}{\partial x}\left[\left(\frac{\partial \tau}{\partial x}\right )^2 + \left (\frac {\partial \tau}{\partial y}\right )^2 \right][/tex]

I worked through that and came up with the same answer, but without the factor of 1/2. Can anyone tell me where it comes from?

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# Homework Help: Partial derivatives question

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