Corpus ID: 14622117

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

@article{Jahnel2010WhenIT,
  title={When is the (co)sine of a rational angle equal to a rational number},
  author={J. Jahnel},
  journal={arXiv: History and Overview},
  year={2010}
}
  • J. Jahnel
  • Published 2010
  • Mathematics
  • arXiv: History and Overview
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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