Let f(x) = sin4x + cos4 x. Then f is an increasing function in the interval :

Question

Let f(x) = sin⁴x + cos⁴ x. Then f is an increasing function in the interval :

$] 0, \frac{\pi}{4}[$

$] \frac{\pi}{4}, \frac{\pi}{2}[$

$] \frac{\pi}{2}, \frac{5 \pi}{8}[$

$] \frac{5 \pi}{8}, \frac{3 \pi}{4}[$

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