# Let $X = x i+ y j+z k$, and $r=||X||$. Prove that $\nabla (\frac{1}{r})=-\frac{X}{r^3}.$

The question is pretty much stated in the title:

Let $X = x i+ y j+z k$, and   $r=||X||$. Prove that $\nabla (\frac{1}{r})=-\frac{X}{r^3}.$

Please include details so I can understand the steps.