Given as f(x) = tan–1(sin x + cos x)
As given
x ∈ (π/4,π/2)
cos x - sin x < 0; here cosine values are smaller than sine values for same angle
((cos x - sin x)/2(1 + sin x cos x)) < 0
f'(x) < 0
Thus, condition for f'(x) to be decreasing
Hence, f(x) is decreasing on interval (π/4,π/2)
