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Introduction of Trigonometry — Maths STD 10 — Question

CBSE BoardEnglish MediumSTD 10MathsIntroduction of Trigonometry1 Mark
MCQ
If $2\cos3\theta=1$ then $\theta=?$
  • A
    $10^\circ$
  • B
    $15^\circ$
  • ✓
    $20^\circ$
  • D
    $30^\circ$
✓

Answer

Correct option: C.
$20^\circ$
$2\cos3\theta=1$
$\Rightarrow\cos3\theta=\frac{1}{2}$
$\Rightarrow\cos3\theta=\cos60^\circ$
$\Rightarrow3\theta=\cos60^\circ$
$\Rightarrow\theta=20^\circ$

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