trigonometry

Question: If $5\cos 2\theta +2{{\cos }^{2}}\frac{\theta }{2}+1=0,-\pi <\theta <\pi $, then $\theta =$

1) $\frac{\pi }{3}$

2) $\frac{\pi }{3},{{\cos }^{-1}}\frac{3}{5}$

3) ${{\cos }^{-1}}\frac{3}{5}$

4) $\frac{\pi }{3},\pi -{{\cos }^{-1}}\frac{3}{5}$

Solution:

Explanation: No Explanation

Solution of trigonometric equations

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