ctg a= 12/5 п

π < a < 3 * π/2 (III четверть). Ctga = 12/5. Тогда tga = 1/ctga = 1/(12/5) = 5/12. Sin^2a = 1/(1 + ctg^2a) = 1 / (1 + 144/25) = 1/(169/25) = 25/169. Sina = -5/13. Cos^2a = 1 – Sin^2a = 1 – 25/169 = 144/169; Cosa = -12/13.