sec−1 (sin x) is real, if
x∈(-∞, ∞)
x∈[-1, 1]
x=(2n+1)π/2, n∈I
x=nπ, n∈I
sec−1(sin x) is real, if sin x≤−1 or sin x≥1 But −1≤sin x≤1 ∴sec−1(sin x) is real only if sin x=−1 or 1⇒x=(2n+1)π2,n ϵ I