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In my electrodynamics course, there's a "maths" introduction and there's something i don't get !

## Homework Statement

It says that :

the integral on the surface of a sphere is

∫1/r

**da**= 4π

**r'**/3 with r=|

**r'**-

**R**|,

**R**the vector from the element da to the center.

**r'**=r'*

**z^**

## The Attempt at a Solution

For me,

**R**=R*

**z^**

So, |

**r'**-

**R**|= (r'²-R²+2r'*R)

^{1/2}

∫1/r

**da**= ∫1/((r'²-R²+2r'*R)

^{1/2}))

**da**

And

**da**= R²*cos dθ dphi

**z^**

And then i'm stuck !

thanks