 # Double Angle Identities

Video lessons with examples and solutions to help PreCalculus students learn to derive the double angle identities and to use them to prove identities.

sin 2A = 2 sin A cos A
cos 2A = cos2 A − sin2 A, cos 2A = 2cos2 A − 1, cos 2A = 1 − 2sin2 A
tan 2A = 2 tan A / (1 − tan2 A)

How to Understand Double Angle Identities Based on the sum formulas for trig functions, double angle formulas occur when alpha and beta are the same. Take a look at how to simplify and solve different double-angle problems that might occur on your test.

This video shows you the basics of Double Angle Trig Formulas. It will show you where they come from and how to use them.
sin 2A = 2 sin A cos A
cos 2A = cos2 A − sin2 A, cos 2A = 2cos2 A − 1, cos 2A = 1 − 2sin2 A
This video shows you how to use double angle formulas to prove identities as well as derive and use the double angle tangent identity.
tan 2A = 2 tan A / (1 − tan2 A)

The derivation of the double angle identities for sine and cosine, followed by some examples.

Using Double Angle Identities to Solve Equations, Example 1.
This video uses some double angle identities for sine and/or cosine to solve some equations.

Using Double Angle Identities to Solve Equations, Example 2.
This video uses some double angle identities for sine and/or cosine to solve some equations.

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