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## Homework Statement

Using Cartesian coordinates show that [tex]\Phi = 1 / r[/tex] satisfies [tex]\nabla^2\Phi=0[/tex] for r not equals to 0.

## Homework Equations

## The Attempt at a Solution

I tried to let [tex]r = x+y+z[/tex], then [tex]\nabla \Phi = ( \frac{1}{r^2} , \frac{1}{r^2}, \frac{1}{r^2})[/tex], but [tex] \nabla . \nabla \Phi [/tex]is obviously not zero