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 P: 25 My problem is that I'm confused about a hint I was given in this problem. I usually use the law of cosine to find the length of $\vec{r}-\vec{r'}$. But the hint here says that I should make it $[r^2 + (z - z_0)^2]^{1/2}$ Where does this come from? I can't quite get my head around the geometrical idea of this hint. Can't the law of cosine be used here?