Answer:
[tex]-\frac{1}{4}(\sqrt{2}+\sqrt{6})[/tex]
Step-by-step explanation:
[tex]cos(\frac{11\pi}{12})[/tex]
[tex]cos(\frac{2\pi}{3}+\frac{\pi}{4})[/tex]
[tex]cos(\frac{2\pi}{3})cos(\frac{\pi}{4})-sin(\frac{2\pi}{3})sin(\frac{\pi}{4})[/tex]
[tex](-\frac{1}{2})(\frac{\sqrt{2}}{2})-(\frac{\sqrt{3}}{2})(\frac{\sqrt{2}}{2})[/tex]
[tex]-\frac{\sqrt{2}}{4}- \frac{\sqrt{6}}{4}[/tex]
[tex]-\frac{1}{4}(\sqrt{2}+\sqrt{6})[/tex]
Helpful Tips:
Sum Identity: [tex]cos(\alpha+\beta)=cos(\alpha)cos(\beta)-sin(\alpha)sin(\beta)[/tex]
