1) cos(2π/3); 2) cos(11π/6); 3) cos 135°; 4) sin π; 5) cos(-π/6); 6) cos 5π;

Решение:

1. \( \cos(\frac{2\pi}{3}) = - \frac{1}{2} \)
2. \( \cos(\frac{11\pi}{6}) = \cos(2\pi - \frac{\pi}{6}) = \cos(\frac{\pi}{6}) = \frac{\sqrt{3}}{2} \)
3. \( \cos(135^{\circ}) = \cos(180^{\circ} - 45^{\circ}) = - \cos(45^{\circ}) = -\frac{\sqrt{2}}{2} \)
4. \( \sin(\pi) = 0 \)
5. \( \cos(-\frac{\pi}{6}) = \cos(\frac{\pi}{6}) = \frac{\sqrt{3}}{2} \)
6. \( \cos(5\pi) = \cos(\pi + 4\pi) = \cos(\pi) = -1 \)

Ответ: 1) -1/2; 2) √3/2; 3) -√2/2; 4) 0; 5) √3/2; 6) -1.