x^6 = 64e^(i(pi+2kpi))
xk = 2e^(i(pi+2kpi)/6)=2e^(i(30+60k)
k=0, 1, 2,...,5
a>0:
x0 = 2e^(i(30))
x5 = 2e^(i(-30))
x0x5=4