PART 3：倍角公式(03:07)

倍角公式是複角公式的特例，只要令 \(\alpha  = \beta  = \theta \) 即可
\(\sin 2\theta  = 2\sin \theta \cos \theta \)
\(\cos 2\theta  = {\cos ^2}\theta  - {\sin ^2}\theta \)
\( = 2{\cos ^2}\theta  - 1\)
\( = 1 - 2{\sin ^2}\theta \)