сos11pi/12+ cos5pi/12

[latex]\cos \frac{11\pi}{12} +\cos \frac{5\pi}{12}=\cos ( \pi -\frac{\pi}{12}) +\cos (\frac{\pi}{2}-\frac{\pi}{12})=-\cos \frac{\pi}{12}+\sin\frac{\pi}{12}= [/latex] [latex]=-\sqrt{2}\sin (\frac{\pi}{4}-\frac{\pi}{12})=-\sqrt{2}\sin \frac{\pi}{6}= -\frac{ \sqrt{2}}{2} [/latex]