How do I use DeMoivre's theorem to solve z^3-1=0z31=0?

2 Answers
Apr 4, 2015

If z^3-1=0z31=0, then we are looking for the cubic roots of unity, i.e. the numbers such that z^3=1z3=1.

If you're using complex numbers, then every polynomial equation of degree kk yields exactly kk solution. So, we're expecting to find three cubic roots.

De Moivre's theorem uses the fact that we can write any complex number as \rho e^{i \theta}= \rho (\cos(\theta)+i\sin(\theta))ρeiθ=ρ(cos(θ)+isin(θ)), and it states that, if
z=\rho (\cos(\theta)+i\sin(\theta))z=ρ(cos(θ)+isin(θ)), then
z^n = \rho^n (\cos(n \theta)+i\sin(n \theta))zn=ρn(cos(nθ)+isin(nθ))

If you look at 11 as a complex number, then you have \rho=1ρ=1, and \theta=2\piθ=2π. We are thus looking for three numbers such that \rho^3=1ρ3=1, and 3\theta=2\pi3θ=2π.

Since \rhoρ is a real number, the only solution to \rho^3=1ρ3=1 is \rho=1ρ=1. On the other hand, using the periodicity of the angles, we have that the three solutions for \thetaθ are
\theta_{1,2,3}=\frac{2k\pi}{3}θ1,2,3=2kπ3, for k=0,1,2k=0,1,2.

This means that the three solutions are:

  1. \rho=1, \theta=0ρ=1,θ=0, which is the real number 11.
  2. \rho=1, \theta=\frac{2\pi}{3}ρ=1,θ=2π3, which is the complex number -1/2 + \sqrt{3}/2 i12+32i
  3. \rho=1, \theta=\frac{4\pi}{3}ρ=1,θ=4π3, which is the complex number -1/2 - \sqrt{3}/2 i1232i
Aug 1, 2017

z=1,ω,ω^2

Explanation:

z^3-1=0

z^3=1

We know that any complex number, a+bi, can be written in modulus-argument form, r(cosx+isinx), where r=sqrt(a^2+b^2) and x satisfies sinx=b/r and cosx=a/r.

therefore 1 =1(cos0+isin0)

So z^3=cos(0+2kpi)+isin(0+2kpi) rarr Since the solutions to trig equations aren't unique, we need to consider other possibilities.

z=[cos(0+2kpi)+isin(0+2kpi)]^(1/3)

Use de Moivre's theorem: (cosx+isinx)^k=coskx+isinkx

z=cos(2/3kpi) +i sin(2/3kpi)

Now we must consider every k such that -pi< 2/3kpi ≤ pi

k = 0, z = 1

k = 1, z = cos(2/3pi) + isin(2/3pi) = -1/2+sqrt3/2 i

k= -1, z= cos(-2/3 pi) + i sin (-2/3pi) = -1/2 - sqrt3/2 i

These values are called the cubic roots of unity and are usually written as 1, omega, omega^2.

The fact that omega' = omega^2 can easily be verified.