Constructible polygon, one for which the cosine or sine of each angle has an exact expression in square roots Heptadecagonal construction, giving the exact expression for cos 2π/17 List of trigonometric identities Niven's theorem on rational values of the sine of a rational multiple of π Ptolemy's table of chords Trigonometric functions Trigonometric number, the value of a trigonometric function of a rational multiple of π