1. Вычислить, отметив угол на окружности: a) cos 870°; б) sin (23π/6); в) cos (-17π/3); г) tg (25π/6); д) ctg (-17π/4).

a) cos 870° = cos (870° - 2*360°) = cos (870° - 720°) = cos 150° = cos (180° - 30°) = -cos 30° = -√3/2 б) sin (23π/6) = sin (24π/6 - π/6) = sin (4π - π/6) = sin (-π/6) = -sin (π/6) = -1/2 в) cos (-17π/3) = cos (17π/3) = cos (18π/3 - π/3) = cos (6π - π/3) = cos (-π/3) = cos (π/3) = 1/2 г) tg (25π/6) = tg (24π/6 + π/6) = tg (4π + π/6) = tg (π/6) = √3/3 д) ctg (-17π/4) = -ctg (17π/4) = -ctg (16π/4 + π/4) = -ctg (4π + π/4) = -ctg (π/4) = -1