(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

I'm having hard time seeing that from this picture:

Follows that:

[tex]\phi(\vect{r})=\frac{q}{4\pi\varepsilon_0}\cdot\frac{1}{|\vec{x}|}[/tex]

The thing that puzzles me isn't the equation but this:

[tex]\frac{1}{|\vec{x}|}=\frac{1}{|\vec{r}-\vec{k}|}=\frac{1}{\sqrt{1+r^2-2r\cos\theta}}[/tex]

2. Relevant equations

The lenght of vector (norm):

[tex]||\vec{a}||=\sqrt{a_1^2+a_2^2+a_3^2}[/tex]

3. The attempt at a solution

Now I know why is [tex]|\vec{x}|=|\vec{r}-\vec{k}|[/tex]

That follows from simple subtraction of two vectors. And from triangle I see that [tex]\cos\theta=\frac{|\vec{k}|}{|\vec{r}|}[/tex], so I can get [tex]\vec{k}[/tex] from that, but how do I get that thing under the square? I read a bit in Jacson, that I would need to transform that into polar coordinate system, but how?

Can someone give me detailed expansion of that?

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# Homework Help: Electrostatic potential of unit charge in vacuum

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