Thinking Mathematically (6th Edition)

Published by Pearson
ISBN 10: 0321867327
ISBN 13: 978-0-32186-732-2

Chapter 7 - Algebra: Graphs, Functions, and Linear Systems - 7.6 Modeling Data: Exponential, Logarithmic, and Quadratic Functions - Exercise Set 7.6 - Page 474: 12

Answer

See below:

Work Step by Step

(a) General equation of a parabola on \[y-\text{axis}\]is \[y=a{{x}^{2}}+bx+c\]. Now, for the parabola \[{{x}^{2}}+4x-5\], coefficient of \[{{x}^{2}}\]is 1. As, \[a>0\], thus the parabola opens upwards. (b) Compare the given equation of parabola\[y={{x}^{2}}+4x-5\]with the general equation and obtain the values of \[a,b\]and\[c\]as, \[\begin{align} & a=1 \\ & b=4 \\ & c=-5 \end{align}\] Now,\[x\]coordinate of the vertex is obtained as, \[\begin{align} & x=-\frac{b}{2a} \\ & =-\frac{4}{2\times 1} \\ & =\frac{-4}{2} \\ & =-2 \end{align}\] Now, substitute above \[x\]coordinate in the equation of the parabola to get the\[y\]coordinate of the vertex as, \[\begin{align} & y={{\left( -2 \right)}^{2}}+4\left( -2 \right)-5 \\ & y=4-8-5 \\ & y=-9 \end{align}\] Thus, vertex is\[\left( -2,-9 \right)\]. (c) Substitute\[y=0\]in the parabola equation and solve for\[x\]as, \[\begin{align} & {{x}^{2}}+4x-5=0 \\ & {{x}^{2}}-x+5x-5=0 \\ & x\left( x-1 \right)+5\left( x-1 \right)=0 \\ & \left( x+5 \right)\left( x-1 \right)=0 \end{align}\] Thus, the given parabola cuts the \[x-\text{axis}\]at two points, that is, \[x=-5\]and \[x=1\]. (d) Substitute\[x=0\]in the parabola equation and solve for\[y\]as, \[\begin{align} & y={{x}^{2}}+4x-5 \\ & y=0+0-5 \\ & y=-5 \end{align}\] Thus, the given parabola cuts the\[y\]axis at\[y=-5\].
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