Let f(x)=ecos−1sin(x+π3),then
f(8π9)=e5π18
f(8π9)=e13π18
f(−7π4)=eπ12
f(7π4)=e11π12
f(x)=ecos−1(cos(π2−x−π3))=ecos−1(cos(π6−x))
∴f(8π9)=ecos−1(cos(π6−8π9))=ecos−1(cos(−13π18)) =ecos−1(cos(13π18))=13π18; Similarly find f(−7π4)