Published by:
CGP EDU Academic Team
Published on: August 13, 2026
The function f(x) = sin 4 x + cos 4 x increasing if -
Text Solution
Verified by ExpertsThe correct answer is:
B
Here, ƒ(x) = sin 4 x + cos 4 x
ƒ ′ (x) = 4 sin 3 x . cos x + 4 cos 3 x (– sin x)
ƒ ′ (x) = 4 sin x cos x (sin 2 x – cos 2 x)
ƒ ′ (x) = 2 (sin 2x) (– cos 2x)
ƒ ′ (x) = – sin 4x
Now, ƒ ′ (x) ≥ 0 if sin 4x ≤ 0
⇒ π ≤ 4x ≤ 2 π
⇒ π /4 ≤ x ≤ π /2
Here is only subset of 
Therefore, is the solution.
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