б) ctg (- 3π/5) = 1/tg(- 3π/5) = 1/tg(π + 2π/5) = 1/tg(2π/5) = 1/(sin(2π/5) / cos(2π/5)) = cos(2π/5) / sin(2π/5).
г) tg ( - 9π/5) = tg (π/5) = sin(pi/5) / cos(π/5).
б) ctg (- 3π/5) = 1/tg(- 3π/5) = 1/tg(π + 2π/5) = 1/tg(2π/5) = 1/(sin(2π/5) / cos(2π/5)) = cos(2π/5) / sin(2π/5).
г) tg ( - 9π/5) = tg (π/5) = sin(pi/5) / cos(π/5).