Complex Roots and Transformation Analysis

00:00 10 marks

  • Exam Style - Paper 3
  • HL
  • Tier 3
  • Complex numbers
  • De Moivre's theorem
  • Nth roots
  • Argand diagram transformations

Consider the equation \(z^n = w\) where \(w\) is a given complex number. Solve for all the \(n\)th roots of \(w\) and describe the geometric transformations on the Argand diagram. Discuss any symmetry that the set of solutions exhibits.

Part a

4 minutes 3 marks

Question

Solve the equation \(z^n = w\) for \(z\). Express the general solution in terms of \(|w|\) and \(\arg(w)\).



Hint Hint

Hint

Recall that in polar form, \(z = r(\cos \theta + i\sin \theta)\) and apply De Moivre’s Theorem.

  • Answer Answer

Complete Answer

The general solution is \( z = \sqrt[n]{|w|} \left[ \cos\left(\frac{\arg(w)+2\pi k}{n}\right) + i\sin\left(\frac{\arg(w)+2\pi k}{n}\right) \right]\) for \(k = 0, 1, \ldots, n-1\).

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Part b

3 minutes 3 marks

Question

Summarize the geometric transformations that map the solutions onto the Argand diagram.



Hint Hint

Hint

Focus on the effects of modulus and argument in positioning the roots.

  • Answer Answer

Complete Answer

The solutions are obtained by scaling by \(\sqrt[n]{|w|}\) and rotating by increments of \(\frac{2\pi}{n}\), resulting in equally spaced points on a circle in the Argand diagram.

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Part c

5 minutes 4 marks

Question

Analyze the symmetry of the set of solutions. What kind of symmetry is observed?



Hint Hint

Hint

Consider the rotational symmetry about the origin.

  • Answer Answer

Complete Answer

The set of solutions exhibits rotational symmetry of order \(n\), with each solution obtained by a rotation of \(\frac{2\pi}{n}\) radians from the previous one.

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