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: 66

Answer

$y \geq-\frac{2}{5} x^2-\frac{3}{5} x+1$

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: $(0,1)$ $x$ intercepts: $(-3,0),(1,0)$ \begin{equation} \begin{aligned} & f(x)=a x^2+b x+c \\ & f(0)=c=1 \end{aligned} \end{equation} We determine $a$ and $b$: \begin{equation} \begin{aligned} f(x)& =a x^2+b x+1 \\ f(-3)& =9a-3b+1=0 \\ f(1) & =a+b+1=0 \end{aligned} \end{equation} Now solve the system of equations for the values of $a$ and $b$. \begin{cases} \begin{aligned} 9a-3 b & =-1 \\ a+b & =-1 \end{aligned} \end{cases} Solve for $b$ in terms of $a$. \begin{equation} \begin{aligned} b & =-1-a \\ \end{aligned} \end{equation} Substitute for $b$ in the first eq. and solve. \begin{equation} \begin{aligned} 9 a-3(-1-a) & =-1 \\ 9+a+3 & =-1 \\ 10 a & =-1-3 \\ 10 a & =-4\\ a=\frac{-4}{10}\\ a=-\frac{2}{5}. \end{aligned} \end{equation} and \begin{equation} \begin{aligned} b & =-1-\left(-\frac{2}{5}\right) \\ & =-1+\frac{2}{5} \\ & =-\frac{5-2}{5} \\ & =-\frac{3}{5}. \end{aligned} \end{equation} The equation of the parabola is. $$ f(x) = -\frac{2}{5}x^2-\frac{3}{5}x+1.$$ 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 does not contain this point. We have: $$\begin{aligned} y&=0\\ -\frac{2}{5}x^2-\frac{3}{5}x+1&=-\frac{2}{5}(0^2-\frac{3}{5}(0)+1=1\end{aligned} $$ As $0\leq 1$, we choose the opposite inequality sign, which is "greater or equal to" ($\geq$). The inequality is: $$y\geq -\frac{2}{5}x^2-\frac{3}{5}x+1.$$
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