[tex] g = -\frac{m}{r^3}\vec{r}[/tex]

[tex] r^2 = x^2 + y^2 + z^2 [/tex]

[tex] \vec{r} = ix + jy +kz [/tex]

[tex] \nabla \cdot g = 0 [/tex]

I'm not sure how this was done. Is the product rule used somehow? What happened to the extra power of r? Thanks for any advice.