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: 65

Answer

$y

Work Step by Step

The $y$ intercept and the two points of the $x$ intercepts are clearly visible from the graph. These points can be used to find an equation of the parabola. $y$- intercept: $(0,-12)$ $x$-intercepts: $(-4,0),(3,0)$ \begin{equation} \begin{aligned} & f(x)=a x^2+b x+c \\ & f(0)=c=-12 \end{aligned} \end{equation} \begin{equation} \begin{aligned} f(x)& =a x^2+b x-12 \\ f(-4)& =16 a-4 b-12=0 \\ f(3) & = 9a+3b-12=0 \end{aligned} \end{equation} Now solve the system of equations for the values of $a$ and $b$. \begin{equation} \begin{cases} 16 a-4 b & =12 \\ 9 a+3 b & =12 \end{cases} \end{equation} Solve for $b$ in term of $a$. \begin{equation} \begin{aligned} 3 b & =12-9 a \\ b & =\frac{12-9 a}{3} \\ & =4-3 a \end{aligned} \end{equation} Substitute for $b$ in the first equation and solve. \begin{equation} \begin{aligned} 16 a-4(4-3 a) & =12 \\ 16 a-16+12 a & =12 \\ 28 a & =12+16 \\ 28 a & =28 \\ a & =1. \end{aligned} \end{equation} and \begin{equation} \begin{aligned} b & =4-3(1) \\ & =1. \end{aligned} \end{equation} The equation of the parabola is. $$ f(x) = x^2+x-12.$$ The graph of the quadratic function is sketched as a dashed curve because there is no "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 does not contain this point. We have: $$\begin{aligned} y&=0\\ x^2+x-12&=0^2+0-12=-12\end{aligned} $$ As $0>-12$, we choose the opposite inequality sign, which is "less than" ($<$). The inequality is: $$y
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