Can somebody help me to figure out the answer of that question?

Thanks in advanced...Really appreciate if have any help...

By Using De Moivre’s Theorem, show that :

(i) Cos 6theta = 32 cos^6theta - 48 cos^4theta + 18 cos^2theta -1

(ii) If z = costheta + i sintheta, nÎZ^+, proof that z^n + z^-n = 2 costheta, hence, Cos 4theta = 8 cos^4theta - 4cos2theta -3

(iii) Find the roots of z^3 = -1 + √3 i