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: 59

Answer

A parabola that opens down with $\left\{\begin{array}{ll} \text {Vertex:} & (-1,1)\\ \text {Focus:} & (-1,0)\\ \text {Directrix:} & y=2 \end{array}\right.$

Work Step by Step

Group like terms $(x^{2}+2x)+4y=3$ Complete the square $(x^{2}+2x+1)+4y=3+1$ $(x+1)^{2}=-4y+4$ $(x+1)^{2}=-4(y-1)$ This is the form of the equation of a parabola, $ x^{2}=-4py\qquad$ (opens downward, $p=1$) where $\left\{\begin{array}{ll} \text {Vertex:} & (0,0)\\ \text {Focus:} & (0,-p)=(0,-1)\\ \text {Directrix:} & y=1 \end{array}\right.$ When $x$ is replaced with $(x+1)$, a shift to the left is made. When $y$ is replaced with $(y-1)$, a shift upward is made. $(x,y)\rightarrow(x-1,y+1)$ So the equation $(x+1)^{2}=-4(y-1)$ represents a parabola that opens down with $\left\{\begin{array}{ll} \text {Vertex:} & (0-1,0+1)=(-1,1)\\ \text {Focus:} & (0-1,-1+1)=(-1,0)\\ \text {Directrix:} & y=1+1=2 \end{array}\right.$
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