Precalculus: Mathematics for Calculus, 7th Edition

Published by Brooks Cole
ISBN 10: 1305071751
ISBN 13: 978-1-30507-175-9

Chapter 11 - Section 11.5 - Rotation of Axes - 11.5 Exercises - Page 823: 13

Answer

$x^2-2\sqrt 3 xy-y^2=-4$

Work Step by Step

Here, we have $x'=x \cos \phi+y \sin \phi$ and $y'=-x \sin \phi+y \cos \phi$ and $\phi =30^{\circ}$ The polar equation for x-component along X-axis is given as: $x'=x \cos \phi+y\sin \phi=x \cos 30^{\circ} +y\sin 30^{\circ}\\=\dfrac{x\sqrt 3}{2}+\dfrac{y}{2}$ The polar equation for y-component along Y-axis is given as: $y'=-x \sin \phi+y\cos \phi=-x \sin 30^{\circ}+y\cos 30^{\circ}\\=\dfrac{x}{2}+\dfrac{y\sqrt 3}{2}$ Consider $x'^2+2\sqrt 3x'y'-y'^2=4$ $(\dfrac{x\sqrt 3}{2}+\dfrac{y}{2})^2+2\sqrt 3(\dfrac{x\sqrt 3}{2}+\dfrac{y}{2}) (\dfrac{x}{2}+\dfrac{y\sqrt 3}{2}) -(\dfrac{x}{2}+\dfrac{y\sqrt 3}{2})^2=4$ This gives: $3x^2+y^2+2\sqrt xy-6x^2+4\sqrt 3 xy+6y^2-x^2-3y^2+2\sqrt 3 xy=16$ Thus, $x^2-2\sqrt 3 xy-y^2=-4$
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