标签： 高等数学基础

$\cot 2 \alpha =$ $\frac{\cot ^{2} \alpha – 1}{2 \cot \alpha}$

$\tan 2 \alpha =$ $\frac{2 \tan \alpha}{1 – \tan ^{2} \alpha}$

$\sin ^{2} \alpha =$ $\frac{1 – \cos 2 \alpha}{2}$

$\cos 2 \alpha =$ $2 \cos ^{2} \alpha -$ $1$

$\cos 2 \alpha =$ $1 -$ $2 \sin ^{2} \alpha$

$\cos 2 \alpha =$ $\cos ^{2} \alpha -$ $\sin ^{2} \alpha$

$\sin 2 \alpha =$ $2 \sin \alpha \cos \alpha$

$1 +$ $\cot ^{2} \alpha =$ $\csc ^{2} \alpha$

$1 +$ $\tan ^{2} \alpha =$ $\sec ^{2} \alpha$

$\sin ^{2} \alpha +$ $\cos ^{2} \alpha =$ $1$

$\cot \alpha =$ $\frac{\cos \alpha}{\sin \alpha}$

$\tan \alpha =$ $\frac{\sin \alpha}{\cos \alpha}$

$\begin{cases} & \tan \alpha \cdot \cot \alpha = 1; \\ & \sin \alpha \cdot \csc \alpha = 1; \\ & \cos \alpha \cdot \sec \alpha = 1. \end{cases}$

$\csc A =$ $\frac{b}{a}$

$\sec A =$ $\frac{b}{c}$