PART 1：複角公式(04:14)

兩個角度加減後的三角函數值可以透過複角公式求值
\(\sin (\alpha  + \beta ) = \sin \alpha \cos \beta  + \cos \alpha \sin \beta \)
\(\sin (\alpha  - \beta ) = \sin \alpha \cos \beta  - \cos \alpha \sin \beta \)
\(\cos (\alpha  + \beta ) = \cos \alpha \cos \beta  - \sin \alpha \sin \beta \)
\(\cos (\alpha  - \beta ) = \cos \alpha \cos \beta  + \sin \alpha \sin \beta \)
\(\tan (\alpha  + \beta )\) 可以使用三角函數恆等式 \(\tan (\alpha  + \beta ) = \frac{{\sin (\alpha  + \beta )}}{{\cos (\alpha  + \beta )}}\)
\(\tan (\alpha  + \beta ) = \frac{{\tan \alpha  + \tan \beta }}{{1 - \tan \alpha \tan \beta }}\) ， \(\tan (\alpha  - \beta ) = \frac{{\tan \alpha  - \tan \beta }}{{1 + \tan \alpha \tan \beta }}\)
複角公式可以導出倍角、半角公式，故應該熟記