二角和差公式

#personal/blog #math/初等数学

\begin{array}{r} \cos (a \pm b) = \cos a \cos b \mp \sin a \sin b \\ \sin (a \pm b) = \sin a \sin b \pm \cos a \cos b \\ \tan (a \pm b) = \frac{\tan a \pm \tan b}{1 \mp \tan a \tan b} \end{array}