Precalculus (6th Edition) Blitzer

Published by Pearson
ISBN 10: 0-13446-914-3
ISBN 13: 978-0-13446-914-0

Chapter 6 - Section 6.5 - Complex Numbers in Polar Form; DeMoivre's Theorem - Exercise Set - Page 768: 52

Answer

The division of the complex numbers in the polar form is $2\left( \cos 0{}^\circ +i\sin 0{}^\circ \right)$.

Work Step by Step

Consider the given complex number to write in the polar form, $\begin{align} & {{z}_{1}}=2-2i \\ & {{z}_{2}}=1-i \\ \end{align}$ (I) For a complex number $z=x+iy$, the polar form is given by, $z=r\left( \cos \theta +i\sin \theta \right)$ (II) Here, $x=r\cos \theta \ \text{ and }\ y=r\sin \theta $, dividing the value of y by x, to get $\tan \theta =\frac{y}{x}$ (III) Also, the value of r is called the moduli of the complex number, given by, For ${{z}_{1}}=2-2i$ $\begin{align} & r=\sqrt{{{x}^{2}}+{{y}^{2}}} \\ & r=\sqrt{{{\left( 2 \right)}^{2}}+{{\left( -2 \right)}^{2}}} \\ & r=\sqrt{4+4} \\ & r=2\sqrt{2} \\ \end{align}$ $r=2\sqrt{2}$ (IV) For any complex number $z=x+iy$, the sign of the value of x and y determine in which quadrant the value of $z=x+iy$ would lie, If the value of x is positive and the value of y is positive, then the angle $\theta $ lies in the first quadrant having the value of $\theta $ as $\theta $ (V) Also, if the value of x is negative and the value of y is positive, then the angle $\theta $ lies in the second quadrant having the value of $\theta $ as $\pi -\theta $ (VI) Also, if the value of x is negative and the value of y is negative, then the angle $\theta $ lies in the third quadrant having the value of $\theta $ as $\pi +\theta $ (VII) Also, if the value of x is positive and the value of y is negative, then the angle $\theta $ lies in the fourth quadrant having the value of $\theta $ as $2\pi -\theta $ (VIII) For the given complex number, using (I), (III) to get, $\begin{align} & \tan \theta =\frac{y}{x} \\ & \tan \theta =\frac{-2}{2} \\ & \tan \theta =-1 \\ \end{align}$ For the given complex number, Using (V), $\theta =\frac{7\pi }{4}$ (IX) Using (II), (IV), and (IX), to get, ${{z}_{1}}=2\sqrt{2}\left( \cos \frac{7\pi }{4}+i\sin \frac{7\pi }{4} \right)$ The polar form of the complex number is ${{z}_{1}}=2\sqrt{2}\left( \cos \frac{7\pi }{4}+i\sin \frac{7\pi }{4} \right)$ For ${{z}_{2}}=1-i$ $\begin{align} & r=\sqrt{{{x}^{2}}+{{y}^{2}}} \\ & r=\sqrt{{{\left( 1 \right)}^{2}}+{{\left( -1 \right)}^{2}}} \\ & r=\sqrt{1+1} \\ & r=\sqrt{2} \\ \end{align}$ $r=\sqrt{2}$ (X) For the given complex number, using (I), (III) to get, $\begin{align} & \tan \theta =\frac{y}{x} \\ & \tan \theta =\frac{-1}{1} \\ & \tan \theta =-1 \\ \end{align}$ For the given complex number, Using (VIII), $\theta =\frac{7\pi }{4}$ (XI) Using (II), (IV), and (IX), to get, ${{z}_{2}}=\sqrt{2}\left( \cos \frac{7\pi }{4}+i\sin \frac{7\pi }{4} \right)$ The polar form of the complex number is ${{z}_{2}}=\sqrt{2}\left( \cos \frac{7\pi }{4}+i\sin \frac{7\pi }{4} \right)$ Consider any complex number, given by, $\begin{align} & {{z}_{1}}={{r}_{1}}\left( \cos {{\theta }_{1}}+i\sin {{\theta }_{1}} \right) \\ & {{z}_{2}}={{r}_{2}}\left( \cos {{\theta }_{2}}+i\sin {{\theta }_{2}} \right) \\ \end{align}$ For a complex number in polar form, the division is calculated as, $\frac{{{z}_{1}}}{{{z}_{2}}}=\frac{{{r}_{1}}}{{{r}_{2}}}\left( \cos \left( {{\theta }_{1}}-{{\theta }_{2}} \right)+i\sin \left( {{\theta }_{1}}-{{\theta }_{2}} \right) \right)$ (XII) The polar form after the division of the complex numbers, $\begin{align} & {{z}_{1}}=2\sqrt{2}\left( \cos \frac{7\pi }{4}+i\sin \frac{7\pi }{4} \right) \\ & {{z}_{2}}=\sqrt{2}\left( \cos \frac{7\pi }{4}+i\sin \frac{7\pi }{4} \right) \\ \end{align}$ (XIII) Divide it using (XII) and (XIII), to get, $\begin{align} & \frac{{{z}_{1}}}{{{z}_{2}}}=\frac{{{r}_{1}}}{{{r}_{2}}}\left( \cos \left( {{\theta }_{1}}-{{\theta }_{2}} \right)+i\sin \left( {{\theta }_{1}}-{{\theta }_{2}} \right) \right) \\ & \frac{{{z}_{1}}}{{{z}_{2}}}=\frac{2\sqrt{2}}{\sqrt{2}}\left( \cos \left( \frac{7\pi }{4}-\frac{7\pi }{4} \right)+i\sin \left( \frac{7\pi }{4}-\frac{7\pi }{4} \right) \right) \\ & =2\left( \cos 0{}^\circ +i\sin 0{}^\circ \right) \end{align}$ The division of the complex numbers in the polar form is $2\left( \cos 0{}^\circ +i\sin 0{}^\circ \right)$
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