- #1

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## Homework Statement

Hi guys, I am have a problem with the question displayed below:

Its 6.1 ii) I am really not sure how I am suppose to approach this. I am new to partials, so any advice would be great.

## Homework Equations

## The Attempt at a Solution

So far I have:

$$\frac{\partial ^2 f}{\partial x^2}=\frac{\partial}{\partial x}\frac{\partial f}{\partial x}=\frac{\partial}{\partial x}\frac{-x}{r^3} $$

using quotient rule:

$$=[\frac{\partial}{\partial x}(-x)(r^3)-(-x)(\frac{\partial}{\partial x}r^3)]/r^6$$

$$\frac{\partial}{\partial x}r^3=3r^2\frac{\partial r}{\partial x}=3r^2*(x/r)=3rx$$

subbing the above into the quotient and simplifying

I get

$$\frac{-r^2+3x^2}{r^5}$$

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