High School Trigonometry/Double-Angle Identities

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\sin{2u} = 2\sin{u}\cos{u}

\cos{2u} = \cos^{2}{u} - \sin^{2}{u}

= 2\cos^{2}{u} - 1

= 1 - 2\sin^{2}{u}

\tan{2u} = \frac{2\tan{u}}{1 - \tan^{2}{u}}

This material was adapted from the original CK-12 book that can be found here. This work is licensed under the Creative Commons Attribution-Share Alike 3.0 United States License