The value of cos−1⁡cos⁡−680∘,is

cos−1⁡cos⁡−680∘=cos−1⁡cos⁡680∘=cos−1⁡cos⁡680×π180=cos−1⁡cos⁡34π9=cos−1⁡cos⁡4π−2π9=cos−1⁡cos⁡2π9=2π9