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Specialist Mathematics · Unit 3 · Further complex numbers · Roots of complex numbers

Determine and examine the 𝑛th roots of complex numbers and their location in the complex plane.

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Question 1

Consider the complex number \( z = 8\operatorname{cis}\left(\frac{2\pi}{3}\right) \). (a) Determine the three cube roots of \( z \), expressing each in the form \( r\operatorname{cis}(\theta) \) where \( 0 \le \theta < 2\pi \). (2 marks) (b) Represent these three roots on an Argand diagram. (1 mark)

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Question 2

Consider the complex number \( z = 8\text{cis}\left(\frac{2\pi}{3}\right) \). (a) Determine all three cube roots of \( z \). Express your answers in polar form \( r\text{cis}(\theta) \), where \( 0 \leq \theta < 2\pi \). (3 marks) (b) Using the table provided, represent each cube root of \( z \) on an Argand diagram and describe the geometric relationship between these roots. (2 marks)

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Question 3

Consider the complex number \(z = 8\operatorname{cis}\left(\frac{2\pi}{3}\right)\). Determine the modulus of each cube root of \(z\). (1 mark) Hence determine the three cube roots of \(z\), expressing your answers in the form \(r\operatorname{cis}(\theta)\). (1 mark)

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Question 4

The diagram shows the complex number \(z = 8\operatorname{cis}\left(\frac{2\pi}{3}\right)\) plotted in the complex plane. (a) Determine all cube roots of \(z\), expressing your answers in the form \(r\operatorname{cis}(\theta)\) where \(0 \leq \theta < 2\pi\). (2 marks) (b) Verify that these roots form the vertices of an equilateral triangle in the complex plane. (1 mark)

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Question 5

The table below shows three complex numbers and their corresponding values of \(n\) for finding \(n\)th roots. **Part (a)** (2 marks) Determine all fourth roots of \(z_1\), expressing your answers in the form \(r\operatorname{cis}(\theta)\). **Part (b)** (3 marks) Determine all cube roots of \(z_3\), expressing your answers in Cartesian form \(x + yi\). Provide an Argand diagram showing the location of these roots as part of your solution.

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Question 6

Let \( z = 8\operatorname{cis}\left(\frac{2\pi}{3}\right) \). (a) Determine the three cube roots of \( z \), expressing your answers in the form \( r\operatorname{cis}(\theta) \) where \( 0 \leq \theta < 2\pi \). (2 marks) (b) Represent these three roots on an Argand diagram. (1 mark)

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Question 7

Consider the complex number \( z = 8\operatorname{cis}\left(\frac{2\pi}{3}\right) \). (a) Determine the three cube roots of \( z \), expressing your answers in the form \( r\operatorname{cis}(\theta) \) where \( -\pi < \theta \leq \pi \). (b) Represent these three cube roots on an Argand diagram.

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Question 8

The diagram shows a complex number z plotted in the complex plane. (a) Determine the three cube roots of z, expressing your answers in the form r cis(θ) where 0 ≤ θ < 2π. (3 marks) (b) Verify that the sum of the three cube roots equals zero. (1 mark)

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Question 9

Given \(z = 8\operatorname{cis}\left(\frac{2\pi}{3}\right)\), what is the principal argument of the principal cube root of \(z\)?

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