Ctg(pix/9)=-1/корень 3

ctg πx/9 = - 1/√3 πx/9 = arcctg (- 1/√3) + πn πx/9 = π - arcctg (1/√3) + πn πx/9 = π - π/3 + πn πx/9 = 2π/3 + πn x = 6 + 9n, n∈Z