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Ptolemy's theorem states that the sum of the products of the lengths of opposite sides is equal to the product of the lengths of the diagonals. When those side-lengths are expressed in terms of the sin and cos values shown in the figure above, this yields the angle sum trigonometric identity for sine: sin(α + β) = sin α cos β + cos α sin β.
The six trigonometric functions are defined for every real number, except, for some of them, for angles that differ from 0 by a multiple of the right angle (90°). Referring to the diagram at the right, the six trigonometric functions of θ are, for angles smaller than the right angle:
The following is a list of integrals (antiderivative functions) of trigonometric functions. For antiderivatives involving both exponential and trigonometric functions, see List of integrals of exponential functions. For a complete list of antiderivative functions, see Lists of integrals.
This definition is valid for all angles, due to the definition of defining x = cos θ and y sin θ for the unit circle and thus x = c cos θ and y = c sin θ for a circle of radius c and reflecting our triangle in the y-axis and setting a = x and b = y. Alternatively, the identities found at Trigonometric symmetry, shifts, and periodicity may
Liouville's theorem (differential algebra) – Says when antiderivatives of elementary functions can be expressed as elementary functions; List of limits; List of mathematical identities; List of mathematical series; Nonelementary integral – Integrals not expressible in closed-form from elementary functions
2.3 Trigonometric, inverse trigonometric, hyperbolic, and inverse hyperbolic functions relationship. ... This list of mathematical series contains formulae for finite ...
A few functions were common historically, but are now seldom used, such as the chord, versine (which appeared in the earliest tables [30]), haversine, coversine, [39] half-tangent (tangent of half an angle), and exsecant. List of trigonometric identities shows more relations between these functions.
The trigonometric functions of angles that are multiples of 15°, 18°, or 22.5° have simple algebraic values. These values are listed in the following table for angles from 0° to 45°. [ 1 ] In the table below, the label "Undefined" represents a ratio 1 : 0. {\displaystyle 1:0.}