Precalculus (6th Edition) Blitzer

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

Chapter 11 - Cumulative Review Exercises - Page 1181: 12

Answer

See below:

Work Step by Step

The standard equation of a hyperbola with center $(h,k)$ and horizontal transverse axis is $\frac{{{\left( x-a \right)}^{2}}}{{{a}^{2}}}-\frac{{{\left( y-k \right)}^{2}}}{{{b}^{2}}}=1$ Consider the equation ${{x}^{2}}-4{{y}^{2}}-4x-24y-48=0$ Now subtract/add $4$ in the equation and rearrange the terms: ${{x}^{2}}-4{{y}^{2}}-4x-24y-48+4-4=0$ Re-arrange the terms $\left( {{x}^{2}}+4-4x \right)-4{{y}^{2}}-24y-48-4=0$ Now subtract $36$ in the equation and rearrange the terms, $\begin{align} & \left( {{x}^{2}}+4-4x \right)-4{{y}^{2}}-24y-36+36-48-4=0 \\ & \left( {{x}^{2}}+4-4x \right)+\left( -4{{y}^{2}}-24y-36 \right)+36-48-4=0 \\ & \left( {{x}^{2}}+4-4x \right)+\left( -4{{y}^{2}}-24y-36 \right)-16=0 \end{align}$ Take out $-4$ and complete the whole square. $\begin{align} & {{\left( x-2 \right)}^{2}}+-4{{\left( {{y}^{2}}+6y+9 \right)}^{2}}=16 \\ & {{\left( x-2 \right)}^{2}}-4{{\left( y+3 \right)}^{2}}=16 \end{align}$ Divide both sides of the equation by $16$ $\begin{align} & \frac{{{\left( x-2 \right)}^{2}}-4{{\left( y+3 \right)}^{2}}}{16}=1 \\ & \frac{{{\left( x-2 \right)}^{2}}}{16}-\frac{{{\left( y+3 \right)}^{2}}}{4}=1 \\ \end{align}$ The last expression can be compared to the general equation of a hyperbola with horizontal transverse axis $\frac{{{\left( x-a \right)}^{2}}}{{{a}^{2}}}-\frac{{{\left( y-k \right)}^{2}}}{{{b}^{2}}}=1$ and center $\left( h,k \right)$. Thus, $\frac{{{\left( x-2 \right)}^{2}}}{{{4}^{2}}}-\frac{{{\left( y+3 \right)}^{2}}}{{{2}^{2}}}=1$ with center $\left( 2,-3 \right)$. To plot the graph, asymptotes are required to be calculated. Asymptotes are given by $ y=k\pm \frac{b}{a}\left( x-h \right)$ For the hyperbola above, the asymptotes are $\begin{align} & y=-3\pm \frac{2}{4}\left( x-2 \right) \\ & y+3=\pm \frac{1}{2}\left( x-2 \right) \\ \end{align}$ Which gives two asymptotes with equation $ y+3=\frac{1}{2}\left( x-2 \right)$ and $ y+3=-\frac{1}{2}\left( x-2 \right)$ Plot these asymptotes. First consider $ y+3=\frac{1}{2}\left( x-2 \right)$ This can be written as $\begin{align} & y+3=\frac{1}{2}\left( x-2 \right) \\ & y=0.5x-1-3 \\ & y=0.5x-4 \\ \end{align}$. This is the same as the graph of the line $ y=x $ but with two transformations. First, horizontally shrink by $0.5$ units and then vertically shift downwards by $4$ units. Now, consider $ y+3=-\frac{1}{2}\left( x-2 \right)$ This can be written as $\begin{align} & y+3=-\frac{1}{2}\left( x-2 \right) \\ & y+3=-0.5x+1 \\ & y=-0.5x-2 \end{align}$. This is the same as the graph of the line $ y=x $ but with two transformations. We horizontally shrink by $-0.5$ units and then vertically shift downwards by $2$ units.
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