When is the (co)sine of a rational angle equal to a rational number?

Details When is the (co)sine of a rational angle equal to a rational number? When is the (co)sine of a rational angle equal to a rational number?

2010-06-15

arxiv.org/abs/1006.2938v1

Mathematics

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Scientific paper

If the cosine of a rational multiple of $\pi$ is a rational number then it is
an integral multiple of $\frac12$. For this fact, we give a proof accessible to
an interested school student. We then discuss which quadratic and cubic
irrationalities are values of cosine at ratinal multiples of $\pi$.

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