cosec−1(cosx) is defined if
x∈[−1, 1]
x∈R
x=(2n+1)π2,n∈Z
x=nπ,n∈Z
We know that cosec−1x is defined for
x∈(−∞, −1]∪[1, ∞). Therefore, cosec−1(cosx) is defined for
cosx=±1⇒x=nπ, n∈Z.