# Double Angle Identities

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]

## References

1. Derive Double Angle Identities (Algebra)
Wumbo (internal)
2. Derive Double Angles Identities (Unit Circle)
Wumbo (internal)
3. Derive Double Angles Identities (Complex Plane)
Wumbo (internal)