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I'm reading Clifford A. Pickover's Math Book, in the Fermat's spiral page, it says the Fermat's spiral formula is $r^2=a^2\theta$, why isn't it written as $r=\pm a\sqrt{\theta}$? What's the problem in writing it that way?

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Aesthetics: polynomial equations are usually nicer to work with. Probably a bit of tradition.

Also, $r = a \sqrt{\theta}$ is only half the spiral anyways: the other half is given by $r = -a \sqrt{\theta}$.

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    Yep, I noticed that when I evaluated it on Mathematica, to be precise, it would be $r=\pm a\sqrt \theta$.2012-10-14
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    Can you comment about **usually nicer to work with**?2012-10-14
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    It's one of those self-evident things that are hard to put words to. Solving for one variable as a function of the other frequently leads to much more complicated formulas, which is a hinderance in the common situation that you don't actually need things expressed that way.2012-10-14
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    Can you provide me an example?2012-10-15
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The standard form works for all $a$ and $\theta$. It also eliminates the need to consider the different branches of the square root.