Another question, I'm really sorry.

2.) Let z=cis(a)

a.) Use De Moivre's theorem to show that z^n + (1/z)^n = 2cos(na)

I was able to prove this part.

b.) Use the binomial theorem to expand (z + (1/z))^4

I got z^4 + 4z^2 +6 +(4/z^2) + (1/z^4), is it right?

Part 2, Hence show that cos^4(a) = (1/8) (cos(4a) + 4cos(2a) + 3 and find the integral of cos^4(a)da.

I'm not sure how to show..

Thank you so so much!

J.