Sinusas kosinusas benrasis kursas

[tex]sin\frac{π}{12}=cosα[/tex]      [tex]α∈0;\frac{π}{2})[/tex]  Apskaičiuokite  [tex]cos3α[/tex]

[tex]\cos\alpha=\sin(\frac{\pi}{2}-\alpha)=\sin\frac{\pi}{12}[/tex]
Kai [tex]\alpha∈(0;\frac{\pi}{2})[/tex], tai:
[tex]\frac{\pi}{2}-\alpha=\frac{\pi}{12}\implies \alpha=\frac{5\pi}{12}[/tex]
[tex]\cos(3\alpha)=\cos(3\cdot \frac{5\pi}{12})=\cos(\frac{5\pi}{4})=\cos(\pi+\frac{\pi}{4})=-\cos\frac{\pi}{4}=-\frac{\sqrt2}{2}[/tex]