Algebra 2 (1st Edition)

Published by McDougal Littell
ISBN 10: 0618595414
ISBN 13: 978-0-61859-541-9

Chapter 9 Quadratic Relations and Conic Sections - 9.2 Graph and Write Equations of Parabolas - 9.2 Exercises - Skill Practice - Page 623: 24

Answer

The parabola, the focus and the directrix should be rotated by $180$ degrees.

Work Step by Step

$\bf{Step\text{ }1}$ Bring the equation in standard form: $$\begin{align*} 0.5y^2+x&=0&&\text{Write the original equation.}\\ 0.5y^2&=-x&&\text{Subtract }x\text { from reach side.}\\ y^2&=-2x&&\text{Multiply both sides by }2. \end{align*}$$ $\bf{Step\text{ }2}$ We identify the focus, directrix and axis of symmetry. The equation has the form $y^2=4px$, where $p=-0.5$. The $\bf{focus}$ is $(p,0)$ or $\left(-0.5,0\right)$. The $\bf{directrix}$ is $x=-p$ or $x=0.5$. Because $y$ is squared, the $\bf{axis\text{ } of\text{ }symmetry}$ is the $x$-axis. $\bf{Step\text{ }3}$ We $\bf{draw}$ the parabola by making a table of values and plotting points. As $p<0$, the parabola opens to the left. So we will use only negative $x$-values. \[ \begin{array}{cccccc} x &|& -1 &|& -2 &|& -3 &|& -4 &|& -5 &|&\\ y &|& \pm 1.41 &|& \pm 2 &|& \pm 2.45 &|& \pm 2.83 &|& \pm 3.16 &|&\\ \end{array}\] The mistake in the given graph is that the parabola, the focus and the directrix should be rotated by $180$ degrees. Here is the correct graph:
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