Thomas' Calculus 13th Edition

Published by Pearson
ISBN 10: 0-32187-896-5
ISBN 13: 978-0-32187-896-0

Chapter 11: Parametric Equations and Polar Coordinates - Section 11.7 - Conics in Polar Coordinates - Exercises 11.7 - Page 686: 50

Answer

$r\displaystyle \cos(\theta+\frac{\pi}{6})=\frac{1}{2}$

Work Step by Step

Conversion formulas: $\left\{\begin{array}{ll} (x,y)=(r\cos\theta,r\sin\theta) & \\ r^{2}=x^{2}+y^{2}, & \tan\theta=\frac{y}{x} \end{array}\right.$ Replace x and y with $ r\cos\theta$ and $ r\sin\theta$ $\sqrt{3}r\cos\theta-r\sin\theta=1$ $r(\sqrt{3}\cos\theta-\sin\theta)=1$ Divide with 2, because $\displaystyle \frac{\sqrt{3}}{2}$ and $\displaystyle \frac{1}{2}$ are possible cosine and sine values. $r(\displaystyle \frac{\sqrt{3}}{2}\cos\theta-\frac{1}{2}\sin\theta)=\frac{1}{2}$ We recognize the angle of $\displaystyle \frac{\pi}{6}$ $r(\displaystyle \cos\frac{\pi}{6}\cos\theta-\sin\frac{\pi}{6}\sin\theta)=\frac{1}{2}$ We recognize the parentheses from the additive identity for cosine, $\cos(\alpha+\beta)=\cos\alpha\cos\beta-\sin\alpha\sin\beta$ The polar equation is $r\displaystyle \cos(\theta+\frac{\pi}{6})=\frac{1}{2}$
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