This is a really confusing question that I haven't been able to solve:

Normally when I post questions, I write down the working that i've done so far (I hate the people who post on here purely for answers and never intend to solve the questions themselves!) but I really have no idea how to start this.Express the power series:

in closed form.

(Hint: Use the fact for the cube root of unity

Since it wants me to use [tex]z^3=1[tex] this will only give me the first three terms. However, since there's an n number of terms would I need the ?