Trigonometry 7th Edition

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

Chapter 8 - Section 8.3 - Products and Quotients in Trigonometric Form - 8.3 Problem Set - Page 440: 54

Answer

$-8+8i\sqrt {3}$

Work Step by Step

In trigonometric form, $\sqrt {3}+i=2(\cos 30^{\circ}+i\sin 30^{\circ})$ $2i=2(\cos 90^{\circ}+i\sin90^{\circ})$ $1+i=\sqrt 2(\cos45^{\circ}+i\sin45^{\circ})$ Using de Moivre's theorem ($z^{n}=r^{n} cis \,n\theta$ where $z=r\, cis\,\theta$ and $n$ is an integer), we get $(\sqrt {3}+i)^{4}=[2(\cos 30^{\circ}+i\sin 30^{\circ})]^{4}$ $=(2)^{4}[\cos(4\cdot30^{\circ})+i\sin(4\cdot30^{\circ})]$ $=16(\cos120^{\circ}+i\sin120^{\circ})$ $(2i)^{5}=[2(\cos 90^{\circ}+i\sin90^{\circ})]^{5}$ $=(2)^{5}[\cos(5\cdot90^{\circ})+i\sin(5\cdot90^{\circ})]$ $=32(\cos 450^{\circ}+i\sin 450^{\circ})$ $(1+i)^{10}=[\sqrt {2}(\cos 45^{\circ}+i\sin 45^{\circ})]^{10}$ $=(\sqrt {2})^{10}[\cos(10\cdot45^{\circ})+i\sin(10\cdot45^{\circ})]$ $=32(\cos450^{\circ}+i\sin450^{\circ})$ Now, $(\sqrt 3+i)^{4}(2i)^{5}=$ $[16(\cos120^{\circ}+i\sin120^{\circ})][32(\cos 450^{\circ}+i\sin 450^{\circ})]=$ $16\cdot32[\cos(120^{\circ}+450^{\circ})+i\sin(120^{\circ}+450^{\circ})]$ $=512(\cos 570^{\circ}+i\sin570^{\circ})$ $\frac{(\sqrt {3}+i)^{4}(2i)^{5}}{(1+i)^{10}}=\frac{512(\cos 570^{\circ}+i\sin570^{\circ})}{32(\cos450^{\circ}+i\sin450^{\circ})}$ $=\frac{512}{32}[\cos(570^{\circ}-450^{\circ})+i\sin(570^{\circ}-450^{\circ})]$ $=16(\cos120^{\circ}+i\sin120^{\circ})$ which in standard form is $=16(-\frac{1}{2}+i\cdot\frac{\sqrt 3}{2})$ $=-8+8i\sqrt {3}$
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