x^n=-1=cosπ+isinπ
x=cos{[π+2(k-1)π]/n}+isin{[π+2(k-1)π]/n} (k=1,2,3,.,n)
  =cos[(2k-1)π/n]+isin[(2k-1)π/n] (k=1,2,3,.,n)
还是发图吧