Cosee−1Cos xis defined if
x∈−1,1
x∈R
x=2n+1π2,n∈I
x=nπ, n∈I
Cosee−1Cos x is defined only if Cosx∈(-∞,-1]∪[1,∞)
⇒cosx=±1 ∵-1≤cosx≤1 ⇒x=nπ, n∈I