# Homework Help: Average of a quantity in all directions

1. Nov 9, 2011

### matteo86bo

1. The problem statement, all variables and given/known data

I have to compute the average of $A$ in all directions for a sphere of radius $R$.
For the direction $r$, $A$ is defined as:

$A=\int_0^R n(r)dr$
where $n(r)$ is the density profile of the sphere.
$\langle A\rangle=\frac{4}{3}\left\langle n \right\rangle R$

2. The attempt at a solution

I don't know how to compute the average over all directions. I mean for direction r the average value of A is simply
$\langle A\rangle_r=\left\langle n \right\rangle R$
so, where this factor 4/3 comes from?

2. Nov 9, 2011

### tjackson3

Well, the way you typically find the average value of some function f(x) on a domain $\Omega$ is the following:

$$<f> = \frac{\int_{\Omega}\ f(x)\ d\Omega}{\int_{\Omega}\ d\Omega$$

Note that the quantity in the denominator is just the area of the domain