\( \Large log \cos x \sin x \ge 2 \) and  \( \Large x \epsilon \left[ 0, 3 \pi \right] \), then \( \Large \sin x \) lines in the interval:

A) \( \Large \left[ 0, \frac{1}{2} \right] \)

B) \( \Large \left[\frac{\sqrt{5}-1}{2}, 1 \right] \)

C) \( \Large \left[ 0, \frac{\sqrt{5}-1}{2} \right] \)

D) none of these

Correct answer:
C) \( \Large \left[ 0, \frac{\sqrt{5}-1}{2} \right] \)

Description for Correct answer:

\( \Large log \cos x \sin x \ge 2 => \sin x \le \cos^{2}x \)

=>\( \Large sinx \le 1 - sin^{2}x \) 

=> \( \Large \sin^{2}x + \sin x - 1 \le 0 \)

=> \( \Large \left(\sin x + \frac{1}{2}\right)^{2} - \frac{5}{4} \le 0 \)

Also by definition of logarithm

\( \Large \sin x > 0, \cos x > 0, \cos x ≠ 1 \)

=> \( \Large \sin x + \frac{1}{2} \le \frac{\sqrt{5}}{2} \)

=> \( \Large 0 < \sin x \le \frac{\sqrt{5}-1}{2} \)



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