Complex Numbers - Roots of Unity

Complex Numbers - Roots of Unity

IB Year 6 | Grade 12

The cube roots of unity are \(z_0=1\), \(z_1=-\tfrac12+\tfrac{\sqrt3}{2}i\) and \(z_2=-\tfrac12-\tfrac{\sqrt3}{2}i\). Their properties follow as below.

  1. \(z_1\) and \(z_2\) are conjugates. So, \(z_2\) is a reflection of \(z_1\) in the real axis.
  2. if \(\omega\) is a cube root of unity, then i.e. \(\omega^3 = 1\)
    1. \(\omega^m\) is also a cube root of unity, for any positive integral value of \(m\). \((\omega^m)^3 = \omega^{3m} = (\omega^3)^m = 1^m = 1\)
    2. If also \(\omega\ne1\), then \((\omega-1)(1+\omega+\omega^2)=\omega^3-1=0\), so \(1+\omega+\omega^2=0\). For \(\omega=1\), the sum is \(3\).
  3. A calculator with a complex polynomial solver can solve \(z^3-1=0\). Verify the three roots algebraically; menu names depend on the model.
  4. the cube roots of unity are represented by the vertices of an equilateral triangle inscribed in a unit circle with centre at the origin, with \(z_0\) on the real axis, and \(z_0\), \(z_1\) and \(z_2\) separated by equal angles of \(\frac{2\pi}{3}\).

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