Question

cos x/2 = cos π. find all solutions in the interval [0,2π)?

Answer 1

\[cos(\pi)=-1\] so your equation is
\[cos(\frac{x}{2})=-1\]
one solution is \[\frac{x}{2}=\pi\]
so \[x=2\pi\]
that is the only one in your interval. in fact it is not in your interval, so perhaps there is no solution!