Hey franios.

What do you mean by binomial form? Is this something like (a+b)^n? What you can do is use z = re^(it) where t = 5pi/3 and then you can use the fact that e^(i5pi/3) = (e^(ipi/3))^5 which means (cos(pi/3) + isin(pi/3))^5 = (cos(5pi/3) + isin(5pi/3)).