11) cos ⅓ = -½

11) cos(x/3) = -1/2

x/3 = ±arccos(-1/2) + 2πn, n ∈ Z

x/3 = ±(π - arccos(1/2)) + 2πn, n ∈ Z

x/3 = ±(π - π/3) + 2πn, n ∈ Z

x/3 = ±2π/3 + 2πn, n ∈ Z

x = ±2π + 6πn, n ∈ Z

Ответ: x = ±2π + 6πn, n ∈ Z