Fermat Number
The list in this article is incomplete in at least two senses. First, it is always possible to apply the half-angle formula to find an exact expression for the cosine of one-half of any angle on the list, then half of that angle, etc. Second, this article exploits only the first two of five known Fermat primes: 3 and 5, whereas algebraic expressions also exist for the cosines of 2π/17, 2π/257, and 2π/65537. In practice, all values of sines, cosines, and tangents not found in this article are approximated using the techniques described at Generating trigonometric tables.
Read more about this topic: Exact Trigonometric Constants
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