(i) Use de Moivre's theorem to show that
\(\cos 4 \theta=\cos ^{4} \theta-6 \cos ^{2} \theta \sin ^{2} \theta+\sin ^{4} \theta\)

(ii) Hence find all the roots of the equation
\(x^{4}-6 x^{2}+1=0\)
in the form \(\tan q \pi\), where \(q\) is a positive rational number.