High School Trigonometry/Double-Angle Identities

$\sin 2u=2\sin u\cos u$

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

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