Mathematics

Problem

The function

f(x) = \sin^4 x + \cos^4 x

increases, if (A)

0 < x < \frac{\pi}{8}

(B)

\frac{\pi}{4} < x < \frac{3\pi}{8}

(C)

\frac{3\pi}{8} < x < \frac{5\pi}{8}

(D)

\frac{5\pi}{8} < x < \frac{3\pi}{4}