- #1
yungman
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For polar coordinates, ##u(x,y)=u(r,\theta)##. Using Chain Rule:
[tex]\frac{\partial u}{\partial x}=\frac{\partial u}{\partial r}\frac{\partial r}{\partial x}+\frac{\partial u}{\partial \theta}\frac{\partial \theta}{\partial x}[/tex]
The book gave
[tex]\frac{\partial ^2 u}{\partial x^2}=\frac{\partial }{\partial x}\left(\frac{\partial u}{\partial x}\right)=\frac{\partial }{\partial r}\left( \frac{\partial u}{\partial r}\frac{\partial r}{\partial x}+\frac{\partial u}{\partial \theta}\frac{\partial \theta}{\partial x}\right)[/tex]
But should it be like this according to the first equation using Chain Rule? That it has to include the ##\theta## part?:
[tex]\frac{\partial ^2 u}{\partial x^2}=\frac{\partial }{\partial r}\left(\frac{\partial u}{\partial x}\right)\frac{\partial r}{\partial x}+\frac{\partial }{\partial \theta}\left(\frac{\partial u}{\partial x}\right)\frac{\partial \theta}{\partial x}[/tex]
I just follow the form like the first equation. If I am wrong, can you explain why?
Thanks
[tex]\frac{\partial u}{\partial x}=\frac{\partial u}{\partial r}\frac{\partial r}{\partial x}+\frac{\partial u}{\partial \theta}\frac{\partial \theta}{\partial x}[/tex]
The book gave
[tex]\frac{\partial ^2 u}{\partial x^2}=\frac{\partial }{\partial x}\left(\frac{\partial u}{\partial x}\right)=\frac{\partial }{\partial r}\left( \frac{\partial u}{\partial r}\frac{\partial r}{\partial x}+\frac{\partial u}{\partial \theta}\frac{\partial \theta}{\partial x}\right)[/tex]
But should it be like this according to the first equation using Chain Rule? That it has to include the ##\theta## part?:
[tex]\frac{\partial ^2 u}{\partial x^2}=\frac{\partial }{\partial r}\left(\frac{\partial u}{\partial x}\right)\frac{\partial r}{\partial x}+\frac{\partial }{\partial \theta}\left(\frac{\partial u}{\partial x}\right)\frac{\partial \theta}{\partial x}[/tex]
I just follow the form like the first equation. If I am wrong, can you explain why?
Thanks
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