Question

If cos $3\theta =\frac{\sqrt{3}}{2};0<\theta <{20}^{\circ }$ then the value of $\theta$ is:

• A. ${15}^{\circ }$
• B. ${10}^{\circ }$
• C. $0^{\circ }$
• D. ${12}^{\circ }$

Answer 1

The correct option is B ${10}^{\circ }$

$\cos 3\theta =\frac{\sqrt{3}}{2}$

$\Rightarrow \cos 3\theta =\cos 30°$

$\Rightarrow 3\theta =30°$

$\Rightarrow \theta =10°$

Hence, the answer is $10°.$