Question:
If $2 \cos 3 \theta=1$ then $\theta=$ ?
(a) 10°
(b) 15°
(c) 20°
(d) 30°
Solution:
As we know that,
$\cos 60^{\circ}=\frac{\sqrt{3}}{2}$
Thus,
if $2 \cos 3 \theta=1$
$\Rightarrow \cos 3 \theta=\frac{1}{2}$
$\Rightarrow 3 \theta=60^{\circ}$
$\Rightarrow \theta=20^{\circ}$
Hence, the correct option is (c).
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