High School Trigonometry/Double-Angle Identities

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

${\begin{aligned}\cos {2u}&=\cos ^{2}{u}-\sin ^{2}{u}\\&=2\cos ^{2}{u}-1\\&=1-2\sin ^{2}{u}\end{aligned}}$

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

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