PART 9：半角公式 例題2

求 \(\cos \frac{5\pi }{8}=?\)

SOL:  

\(\cos \frac{5\pi }{8}=\pm \sqrt{\frac{1+\cos {5\pi }/{4}\;}{2}}=-\sqrt{\frac{1-{\sqrt{2}}/{2}\;}{2}}\) \(=\frac{\sqrt{2-\sqrt{2}}}{2}\)(取負)