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. 


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Erdos Erdos
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