With some simplification,
cos⁻¹(x) - 5 cos⁻¹(x) - π/3 = 2π/3
becomes
-4 cos⁻¹(x) = π
or
cos⁻¹(x) = -π/4
However, the inverse cosine function has a range of 0 ≤ cos⁻¹(x) ≤ π, so there are no solutions to this equation.
