Решение
sin a, tg a, ctg a если:
cos a = - 3/5, a = π/2 < a < 2π.
sina = (+-) √(1 - cos²a) = (+-)√(1 - (-3/5)²) = (+-)√(1 - 9/25) =
√(16/25) = (+-)(4/5)
tga = sina/cosa = (-)4/5 : (-3/5) = 4/3 или tgx = 4/5 : (-3/5) = - 4/3
ctgx = 1/tgx
ctgx = 3/4 или ctgx = - 3/4