If θ∈[4π,5π], then cos−1(cosθ) equals
-4 π+θ
5 π-θ
4 π-θ
θ-5π
We have,
θ∈[4π,5π]⇒−4π+θ∈[0,π]
Also,
cos(−4π+θ)=cos(4π−θ)=cosθ∴ cos−1(cosθ)=cos−1{cos(−4π+θ)ξ=−4π+θ