立方根
− z 3 = − cos  21 ∘ − i sin  21 ∘ 3 = cos  ( − 159 ∘ ) + i sin  ( − 159 ∘ ) 3 = cos  ( − 53 ∘ ) + i sin  ( − 53 ∘ ) = − ω ( cos  7 ∘ + i sin  7 ∘ ) {\displaystyle {\begin{aligned}{\sqrt[{3}]{-z}}={\sqrt[{3}]{-\cos 21^{\circ }-i\sin 21^{\circ }}}=&{\sqrt[{3}]{\cos(-159^{\circ })+i\sin(-159^{\circ })}}\\=&\cos(-53^{\circ })+i\sin(-53^{\circ })\\=&-\omega (\cos 7^{\circ }+i\sin 7^{\circ })\end{aligned}}}