Medium MCQ +4 / -1 PYQ · JEE Mains 2021

The sum of all values of x in [0, 2$\pi$], for which sin x + sin 2x + sin 3x + sin 4x = 0, is equal to :

  1. A 8$\pi$
  2. B 11$\pi$
  3. C 12$\pi$
  4. D 9$\pi$ Correct answer

Solution

$(\sin x + \sin 4x) + (\sin 2x + \sin 3x) = 0$<br><br>$$ \Rightarrow 2\sin {{5x} \over 2}\left\{ {\cos {{3x} \over 2} + \cos {x \over 2}} \right\} = 0$$<br><br>$\Rightarrow 2\sin {{5x} \over 2}\left\{ {2\cos x\cos {x \over 2}} \right\} = 0$<br><br>$$2\sin {{5x} \over 2} = 0 \Rightarrow {{5x} \over 2} = 0,\pi ,2\pi ,3\pi ,4\pi ,5\pi $$<br><br>$$ \Rightarrow x = 0,{{2\pi } \over 5},{{4\pi } \over 5},{{6\pi } \over 5},{{8\pi } \over 5},2\pi $$<br><br>$$\cos {x \over 2} = 0 \Rightarrow {x \over 2} = {\pi \over 2} \Rightarrow x = \pi $$<br><br>$\cos x = 0 \Rightarrow x = {\pi \over 2},{{3\pi } \over 2}$<br><br>So, sum $= 6\pi + \pi + 2\pi = 9\pi$

About this question

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

This question is part of PrepWiser's free JEE Main question bank. 52 more solved questions on Trigonometric Functions are available — start with the harder ones if your accuracy is >70%.

Drill 25 more like these. Every day. Free.

PrepWiser turns these solved questions into a daily practice loop. Chapter-wise drills, full mocks, AI doubt chat. No auto-renew.

Start free →