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## Homework Statement

if [tex]\phi[/tex] =

**r**

**k**/r[tex]^{3}[/tex] where

**r**=x

**i**+ y

**J**+ z

**k**and r is the magnitude of

**r**, prove that [tex]\nabla[/tex][tex]\phi[/tex] = (1/r[tex]^{}5[/tex])(r[tex]^{}2[/tex]

**k**-3(

**r**.

**k**)

**r**

Right i'm not really sure where to start here...

i know

Magnitude r = [tex]\sqrt{x^{2}+y^{2}+z^{2}}[/tex]