CBSE BoardEnglish MediumSTD 11 ScienceMathsTrigonometric Equations3 Marks
Question
Solve the following equations: $\sin\theta+\sin5\theta=\sin3\theta$
✓
Answer
$\sin\text{x}+\sin5\text{x}=\sin3\text{x}$
$\Rightarrow2\sin3\text{x},\cos2\text{x}-\sin3\text{x}=0$
$\Big[\because\sin\text{C}+\sin\text{D}=2\sin\frac{\text{C}+\text{D}}{2}.\cos\frac{\text{C}-\text{D}}{2}\Big] $
Either
$\Rightarrow\sin3\text{x}=0$ or $2\cos2\text{x}-1=0 $
$\Rightarrow3\text{x}=\text{n}\pi,\text{n }\in\ \text{z}$ or $\cos2\text{x}=\frac{1}{2}=\cos\frac{\pi}{3}$
$\Rightarrow\text{x}=\frac{\text{n}\pi}{3},\text{n }\in\ \text{z}$ or $2\text{x}=2\text{m}\pi\pm\frac{\pi}{3},\text{m }\in\ \text{z}$
or $\text{x}=\text{m}\pi\pm\frac{\pi}{6}$
Thus,
$\text{x}=\frac{\text{n}\pi}{3}$ or $\text{m}\pi\pm\frac{\pi}{6},\text{n, m }\in\ \text{z}$
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