Pythagorean trigonometric identity (English Wikipedia)

Analysis of information sources in references of the Wikipedia article "Pythagorean trigonometric identity" in English language version.

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Leff, Lawrence S. (2005). Precalculus the Easy Way (7th ed.). Barron's Educational Series. p. 296. ISBN 0-7641-2892-2.

This result can be found using the distance formula $d={\sqrt {x^{2}+y^{2}}}$ for the distance from the origin to the point $(x,\ y)$ . See Young, Cynthia Y. (2009). Algebra and Trigonometry (2nd ed.). Wiley. p. 210. ISBN 978-0-470-22273-7. This approach assumes Pythagoras' theorem. Alternatively, one could simply substitute values and determine that the graph is a circle.
Hungerford, Thomas W.; Shaw, Douglas J. (2008). "§6.2 The sine, cosine and tangent functions". Contemporary Precalculus: A Graphing Approach (5th ed.). Cengage Learning. p. 442. ISBN 978-0-495-10833-7.
Hamilton, James Douglas (1994). "Power series". Time series analysis. Princeton University Press. p. 714. ISBN 0-691-04289-6.
Krantz, Steven George (2005). "Definition 10.3". Real analysis and foundations (2nd ed.). CRC Press. pp. 269–270. ISBN 1-58488-483-5.
Tyn Myint-U; Debnath, Lokenath (2007). "Example 8.12.1". Linear partial differential equations for scientists and engineers (4th ed.). Springer. p. 316. ISBN 978-0-8176-4393-5.