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 11.6 - Conic Sections - Exercises 11.6 - Page 679: 60

Answer

A parabola that opens to the right: with vertex: $\quad (-2,2)$ and focus: $\quad (0,2)$

Work Step by Step

We group like terms (containing y, as it is squared) $(y^{2}-4y)-8x-12=0$ $(y^{2}-4y)=8x+12$ We complete the square $(y^{2}-4y +4)=8x+12+4$ $(y-2)^{2}=8x+16$ $(y-2)^{2}=8(x+2)$ This has the form of an equation of a parabola, $y^{2}=4px ,\qquad$ (opens right), $p=2$, the focus is 2 units to the right of the vertex at the origin. When $x$ is replaced with $(x+2)$, a shift to the left is made. When $y$ is replaced with $(y-2)$, a shift upward is made. $(x,y)\rightarrow(x-2,y+2)$ So the equation $(y-2)^{2}=8(x+2)$ represents a shift of the center to $(0-2,0+2)= (-2,2)$ . The focus is 2 units to the right, $(0,2)$.
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