Medium MCQ +4 / -1 PYQ · JEE Mains 2024

Let $$|\cos \theta \cos (60-\theta) \cos (60+\theta)| \leq \frac{1}{8}, \theta \epsilon[0,2 \pi]$$. Then, the sum of all $\theta \in[0,2 \pi]$, where $\cos 3 \theta$ attains its maximum value, is :

  1. A $6 \pi$ Correct answer
  2. B $9 \pi$
  3. C $18 \pi$
  4. D $15 \pi$

Solution

<p>$$\begin{aligned} & |\cos \theta \cos (60-\theta) \cos (60+\theta)| \leq \frac{1}{8} \\ & \Rightarrow \frac{1}{4}|\cos 3 \theta| \leq \frac{1}{8} \\ & \cos 3 \theta \text { is max if } \cos 3 \theta=\frac{1}{2} \\ & \therefore \theta=\frac{\pi}{9}, \frac{5 \pi}{9}, \frac{7 \pi}{9}, \frac{11 \pi}{9}, \frac{13 \pi}{9}, \frac{17 \pi}{9} \\ & \sum \theta_i=6 \pi \end{aligned}$$</p>

About this question

Subject: Mathematics · Chapter: Trigonometric Functions · Topic: Trigonometric Ratios and Identities

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