Intermediate Algebra: Connecting Concepts through Application

Published by Brooks Cole
ISBN 10: 0-53449-636-9
ISBN 13: 978-0-53449-636-4

Chapter 4 - Quadratic Functions - 4.7 Graphing Quadratics from Standard Form - 4.7 Exercises - Page 388: 67

Answer

$y\geq-\frac{1}{2}(x+3)^2+2$

Work Step by Step

The vertex and the two points of the $x$ intercepts are clearly visible from the graph. The vertex and one of the points of the $x$ intercepts can be used to find an equation of the parabola. Vertex: $(-3,2)$ $x$-intercepts: $(-5,0),(-1,0)$ Hence \begin{equation} \begin{aligned} f(x) & =a(x-h)^2+k \\ & =a(x+3)^2+2. \end{aligned} \end{equation} We determine $a$: \begin{equation} \begin{aligned} f(-5) & =a(-5+3)^2+2 \\ 0 & =4 a+2 \\ -2 & =4 a \\ -\frac{1}{2} & =a. \end{aligned} \end{equation} The equation of the parabola is. \begin{equation} f(x)=-\frac{1}{2}(x+3)^2+2. \end{equation} The graph of the quadratic function is sketched as a solid curve because there is a "equal to" part. Now we have to find the inequality sign. We take a test point, for example $(0,0)$. The solution is the region which contains this point. We have: $$\begin{aligned} y&=0\\ -\frac{1}{2}(x+3)^2+2&=-\frac{1}{2}(0+3)^2+2=-\frac{5}{2}\end{aligned} $$ As $0\geq -\frac{5}{2}$, we choose the inequality sign, as "greater or equal to" ($\geq$). The inequality is: $$y\geq -\frac{1}{2}(x+3)^2+2.$$
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