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## Homework Statement

There is one thing I'm not getting in this problem. It's about uniformly magnetized sphere. In solving the problem with formula for magnetic potential:

[tex]\phi_M=-\nabla\cdot\int\frac{\vec{M}}{|\vec{r}-\vec{r}'|}d^3r'[/tex]

At one point a change of variable for differentiation is made

[tex]\frac{\partial}{\partial z}=\frac{\partial r}{\partial z}\frac{\partial}{\partial r}[/tex]

And he says that [tex]\frac{\partial r}{\partial z}=\cos\theta[/tex]. But I can't see that. If I'm using the formula for spherical coordinate system transformation: [tex]z=r\cos\theta[/tex] I don't get just cosine, I get 1/cosine :\

So what formula is he using?

Thanks