If $3 \sin \theta=2 \sin 3 \theta$ and $0 < \theta < \pi$, then $\sin \theta=$

If $3 \sin \theta=2 \sin 3 \theta$ and $0 < \theta < \pi$, then $\sin \theta=$
  1. $\frac{\sqrt{2}}{\sqrt{5}}$
  2. $\frac{\sqrt{3}}{2 \sqrt{2}}$
  3. $\frac{\sqrt{2}}{3}$
  4. $\frac{\sqrt{3}}{\sqrt{5}}$

Solution

$\begin{aligned} & 3 \sin \theta=2 \sin 3 \theta \\ & \quad=2\left(3 \sin \theta-4 \sin ^2 \theta\right) \\ & \therefore 8 \sin ^3 \theta-3 \sin \theta=0 \\ & \therefore \sin \theta\left(8 \sin ^2 \theta-3\right)=0 \\ & \therefore \sin \theta=0 \text { or } \sin \theta= \pm \sqrt{\frac{3}{8}}= \pm \frac{\sqrt{3}}{2 \sqrt{2}} \end{aligned}$ Since, $0 < \theta < \pi$, we write $\sin \theta=\frac{\sqrt{3}}{2 \sqrt{2}}$

Asked in: MHT CET 2021 (23 Sep Shift 2)

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