Question 17

Mathematics Trigonometric Equations Hard

The number of the solutions of the equation sin x + sin 5x = sin 3x, lying in the interval [0, π], is:

(A) 4
(B) 6
(C) 5
(D) 2
✓ Correct Answer: Option B Verified Solution

📘 Step-by-Step Solution & Detailed Explanation:

\(\sin x + \sin 5x = 2\sin 3x\cos 2x\). So the equation becomes \(2\sin 3x\cos 2x = \sin 3x\), or \(\sin 3x(2\cos 2x - 1) = 0\).

  • \(\sin 3x = 0\): \(x = 0, \frac{\pi}{3}, \frac{2\pi}{3}, \pi\). That's 4 solutions.
  • \(\cos 2x = \frac{1}{2}\): \(2x = \frac{\pi}{3}\) or \(\frac{5\pi}{3}\), so \(x = \frac{\pi}{6}, \frac{5\pi}{6}\). That's 2 solutions.

Total = 6.

Answer: (B) 6

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