Trigonometry 7th Edition

Published by Cengage Learning
ISBN 10: 1111826854
ISBN 13: 978-1-11182-685-7

Chapter 8 - Section 8.4 - Roots of a Complex Number - 8.4 Problem Set - Page 447: 18

Answer

The two square roots are: \[ -\frac{\sqrt{6}}{2}+\frac{\sqrt{2}}{2} i \quad \text { , } \quad \frac{\sqrt{6}}{2}-\frac{\sqrt{2}}{2} i \]

Work Step by Step

\[ 1-i \sqrt{3} \] The modulus of a complex number in the standard form $x+y i$ is: \[ \begin{array}{c} \sqrt{x^{2}+y^{2}} =r\\ \sqrt{1^{2}+(-\sqrt{3})^{2}}=r \\ \sqrt{3+1}=r \\ \sqrt{4}=r \\ 2=r \end{array} \] The $\theta$ angle is the least positive angle for which: \[ \frac{1}{2}=\frac{x}{r}=\cos \theta \] , \[ -\frac{\sqrt{3}}{2}=\frac{y}{r}=\sin \theta \] Thus, \[ 300^{\circ}=\theta \] Using the definition of a complex number in trigonometric form: \[ \begin{aligned} &=(\cos \theta+i \sin \theta)r \\ =\left(\cos 300^{\circ}+i \sin 300^{\circ}\right)2=z \end{aligned} \] Using the roots theorem: \[ r^{\frac{1}{n}}\left[\cos \left(\frac{\theta}{n}+\frac{360^{\circ}}{n} k\right)+i \sin \left(\frac{\theta}{n}+\frac{360^{\circ}}{n} k\right)\right]=w_{k} \] When $0=k$ : \[ \begin{array}{c} 2 \frac{1}{2}\left[\cos \left(\frac{300^{\circ}}{2}+\frac{360^{\circ}}{2}(0)\right)+i \sin \left(\frac{300^{\circ}}{2}+\frac{360^{\circ}}{2}(0)\right)\right]=u_{0} \\ =\left[\cos 150^{\circ}+i \sin 150^{\circ}\right]\sqrt{2} \\ =\left[\frac{-\sqrt{3}}{2}+\frac{1}{2} i\right]\sqrt{2} \end{array} \] Multiply: \[ =-\frac{\sqrt{6}}{2}+\frac{\sqrt{2}}{2} i \] When $1=k$ : \[ \begin{array}{c} 2^{\frac{1}{2}}\left[\cos \left(\frac{300^{\circ}}{2}+\frac{360^{\circ}}{2}(1)\right)+i \sin \left(\frac{300^{\circ}}{2}+\frac{360^{\circ}}{2}(1)\right)\right]=w_{1} \\ =\sqrt{2}\left[\cos \left(150^{\circ}+180^{\circ}\right)+i \sin (180+150)\right] \\ =\left[\cos 330^{\circ}+i \sin 330^{\circ}\right] \sqrt{2}\\ =\left[\frac{\sqrt{3}}{2}-\frac{1}{2}\right]\sqrt{2} \end{array} \] Multiply: \[ =\sqrt{\frac{\sqrt{6}}{2}-\frac{\sqrt{2}}{2} i} \]
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