The double angle identities are trigonometric identities that give the cosine and sine of a double angle in terms of the cosine and sine of a single angle.
Two additional double angle identities are sometimes given that combine the pythagorean identity and the cosine identity above using algebra.
The double identities can be derived a number of ways:
- Using the sum of two angles identities and algebra[1]
- Using the inscribed angle theorem and the unit circle[2]
- Using the the trigonometry of the complex plane[3]
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Derive Double Angle Identities (Algebra) Example
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Derive Double Angles Identities (Unit Circle) Example
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Derive Double Angles Identities (Complex Plane) Example