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These identities are useful whenever expressions involving trigonometric functions need to be simplified. An important application is the integration of non-trigonometric functions: a common technique involves first using the substitution rule with a trigonometric function, and then simplifying the resulting integral with a trigonometric identity.
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 abundance of the theorems and identities inherited from Fourier series make the Chebyshev polynomials important tools in numeric analysis; for example they are the most popular general purpose basis functions used in the spectral method, [16] often in favor of trigonometric series due to generally faster convergence for continuous functions ...
Visual proof of the Pythagorean identity: for any angle , the point (,) = (, ) lies on the unit circle, which satisfies the equation + =.Thus, + =. In mathematics, an identity is an equality relating one mathematical expression A to another mathematical expression B, such that A and B (which might contain some variables) produce the same value for all values of the variables ...
Proof without words of the sum-and-difference-to-product cosine identity using an isosceles triangle – x is actually sin a sin b. The trigonometric identities exploited by prosthaphaeresis relate products of trigonometric functions to sums. They include the following:
This list of mathematical series contains formulae for finite and infinite sums. It can be used in conjunction with other tools for evaluating sums. Here, is taken to have the value
Trigonometric functions were among the earliest uses for mathematical tables. [48] Such tables were incorporated into mathematics textbooks and students were taught to look up values and how to interpolate between the values listed to get higher accuracy. [49] Slide rules had special scales for trigonometric functions. [50]
The proof of the angle sum identities by Euler's formula is not valid because it creates circular dependency. All 3 proofs of Euler's formula (power series, calculus, differential equations) rely on the derivatives of the trigonometric functions, which in turn rely on the angle sum identities to simplify sin(x+h) and cos(x+h).