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## Homework Statement

Every time I pick up Jackson I feel dumb :(

So in 5.5 chapter about Circular Current Loop, he says:

[tex]|\vec{r}-\vec{r}'|=[r^2+r'^2-2rr'(\cos\theta\cos\theta'+\sin\theta\sin\theta'\cos\phi')]^{1/2}[/tex]

So it's cosine rule, but how did he got to that sine/cosine thing? It should be the angle between r and r'. I tried drawing but to no avail :\

help...