Precalculus (6th Edition) Blitzer

Published by Pearson
ISBN 10: 0-13446-914-3
ISBN 13: 978-0-13446-914-0

Chapter 2 - Section 2.2 - Quadratic Functions - Exercise Set - Page 332: 87

Answer

The graph is shown below:

Work Step by Step

Let us consider the parabola defined by the quadratic equation $f\left( x \right)=5{{x}^{2}}+40x+600$ The coefficient of ${{x}^{2}}$ is $5$. Therefore, the value of $a$ is positive and the parabola opens in the upward direction. Compare the equation with the standard form of the quadratic equation $f\left( x \right)=a{{x}^{2}}+bx+c$. So, $\begin{align} & a=5 \\ & b=40 \\ & c=600 \end{align}$ The general form of the parabola vertex is $\left( -\frac{b}{2a},f\left( -\frac{b}{2a} \right) \right)$. Putting in the values of $a$ and $b$ to find the $x\text{-}$ coordinate of the vertex, we get: $\begin{align} & x=-\frac{b}{2a} \\ & =-\frac{40}{2\left( 5 \right)} \\ & =-\frac{40}{\left( 10 \right)} \\ & =-4 \end{align}$ Putting in the value of $x$ in the equation to find the $y\text{-}$ coordinate of the vertex, we get: $\begin{align} & f\left( x \right)=5{{x}^{2}}+40x+600 \\ & f\left( -4 \right)=5{{\left( -4 \right)}^{2}}+40\left( -4 \right)+600 \\ & =80-160+600 \\ & =520 \end{align}$ So, the vertex of the parabola is $\left( -\frac{b}{2a},f\left( -\frac{b}{2a} \right) \right)=\left( -4,520 \right)$. Use a graphing utility to plot the graph: Step 1: Write the function. Step 2: Set the window $x:\left( -15,90,15 \right)$ and $y:\left( 0,5000,500 \right)$. Step 3: Plot the graph.
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