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 - Guided Practice for Examples 1 and 2 - Page 622: 3

Answer

Focus: $\left(0,-1\right)$ Directrix: $y=1$ Axis of symmetry: $y$-axis

Work Step by Step

$\bf{Step\text{ }1}$ Bring the equation in standard form: $$\begin{align*} y&=-\dfrac{1}{4}x^2&&\text{Write the original equation.}\\ -4y&=x^2&&\text{Multiply by }-4. \end{align*}$$ $\bf{Step\text{ }2}$ We identify the focus, directrix and axis of symmetry. The equation has the form $x^2=4py$, where $p=-1$. The $\bf{focus}$ is $(0,p)$ or $\left(0,-1\right)$. The $\bf{directrix}$ is $y=-p$ or $y=1$. Because $x$ is squared, the $\bf{axis\text{ } of\text{ }symmetry}$ is the $y$-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 downward. \[ \begin{array}{cccccc} x &|& \pm 1 &|& \pm 2 &|& \pm 3 &|& \pm4 &|& \pm 5 &|&\\ y &|& -0.25 &|& -1 &|& -2.25 &|& -4 &|& -6.25 &|&\\ \end{array}\]
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