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: 13

Answer

See below:

Work Step by Step

(a) The general equation of a parabola on \[y-\text{axis}\]is \[y=a{{x}^{2}}+bx+c\]. Now, for the parabola \[-{{x}^{2}}+4x-3\], coefficient of \[{{x}^{2}}\]is \[-1\]. As, \[a<0\], thus the parabola opens downwards. (b) Compare the given equation of the parabola\[y=-{{x}^{2}}+4x-3\]with the general equation and obtain the values of \[a,b\]and\[c\]as \[\begin{align} & a=-1 \\ & b=4 \\ & c=-3 \end{align}\] Now, the \[x\]coordinate of the vertex is obtained as, \[\begin{align} & x=-\frac{b}{2a} \\ & =-\frac{4}{2\times \left( -1 \right)} \\ & =\frac{4}{2} \\ & =2 \end{align}\] Now, substitute above \[x\]coordinate in the equation of the parabola to get the\[y\]coordinate of the vertex as, \[\begin{align} & y=-{{\left( 2 \right)}^{2}}+4\left( 2 \right)-3 \\ & y=-4+8-3 \\ & y=1 \end{align}\] Thus, the vertex is\[\left( 2,1 \right)\]. (c) Substitute\[y=0\]in the parabola equation and solve for\[x\]as \[\begin{align} & -{{x}^{2}}+4x-3=0 \\ & {{x}^{2}}-4x+3=0 \\ & {{x}^{2}}-x-3x-3=0 \\ & x\left( x-1 \right)-3\left( x-1 \right)=0 \end{align}\] Solve ahead to get the factors as \[\left( x-3 \right)\left( x-1 \right)=0\] Thus, the given parabola cuts the \[x-\text{axis}\]at two points, that is, \[x=3\]and \[x=1\]. (d) Substitute\[x=0\]in the parabola equation and solve for\[y\]as \[\begin{align} & y=-{{x}^{2}}+4x-3 \\ & y=0+0-3 \\ & y=-3 \end{align}\] Thus, the given parabola cuts the\[y\]axis at\[y=-3\].
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