Find the principal solution of $\cos x=-\frac{1}{2}$

Find the principal solution of $\frac{1+cosx}{cosx}=2$

Find the principal solutions of cos 3x - cos 2x + cos x = 0

Find the principal solutions of sin 2x + cos x = 0

Find the principal solutions of each of the following equations.

(i)sin $x=\frac{1}{2}$

(ii)cos $x=\frac{1}{\sqrt{2}}$

(iii)tan $x=\frac{1}{\sqrt{3}}$