\(-8 = 8(\cos \pi+i\sin \pi)\)
\(-8\) 的三次方根(cube root)有三個,分別是 \(\displaystyle 2\left(\cos \frac{\pi+2k\pi}{3}+i\sin\frac{\pi+2k\pi}{3}\right)\),其中 \(k=0,1,2\)
Mathematica 給的是 principal cube root,也就是 \(\displaystyle 2\left(\cos \frac{\pi}{3}+i\sin\frac{\pi}{3}\right)\)
參閱資料:1.
http://en.wikipedia.org/wiki/Cube_root
2.
http://mathworld.wolfram.com/CubeRoot.html