Согласно формулам приведения
1) cos 5*π/6 = - cos (π - 5*π/6) = - cos π/6 = - √ 3 / 2
2) sin (-7*π/4) = sin (-7*π/4 + 2*π) = sin π/4 = √ 2 / 2
3) tg 11*π/3 = tg(11*π/3 - 4*π) = tg (-π/3) = - tg π/3 = - √ 3
4) ctg (-3,5*π) = ctg (-3,5*π + 4*π) = ctg π/2 = 0