Hello!

(First problem) Express in terms of and

Attempt:According tode Moivre's formula- so:

de Moivre- and

binomial-

So,

(1)

After some arithmetic arrangements and andPythagorean trigonometric identityI got this: and task under a) is completed (I think).

But the problem is at task b), so, I don't know how to write expression (1) in terms of cosx function. Is there any identity? Or maybe some other method.

(Second problem) If is a complex number, what does this equation represent:

Attempt:No attempt. I need a complete solution and explanation.

Thank you!