Precalculus: Mathematics for Calculus, 7th Edition

Published by Brooks Cole
ISBN 10: 1305071751
ISBN 13: 978-1-30507-175-9

Chapter 11 - Section 11.4 - Shifted Conics - 11.4 Exercises - Page 814: 25

Answer

(a) center $(-1,-1)$, vertices $(2,-1), (-4,-1)$, foci $(-1\pm\sqrt {13},-1)$. asymptotes: $y=\frac{2}{3}x-\frac{1}{3}$ and $y=-\frac{2}{3}x-\frac{5}{3}$. (b) See graph.

Work Step by Step

(a) Step 1. From the un-shifted equation $\frac{x^2}{9}-\frac{y^2}{4}=1$, we have $a=3, b=2, c=\sqrt {3^2+2^2}=\sqrt {13}$. We can identify the old center as $(0,0)$, vertices as $(\pm3, 0)$, foci as $(\pm\sqrt {13}, 0)$, and asymptotes as $y=\pm\frac{2}{3}x$ Step 2. With the new equation $\frac{(x+1)^2}{9}-\frac{(y+1)^2}{4}=1$, we can identify the shift as 1 unit to the left and 1 unit down, the new center as $(-1,-1)$, new vertices as $(2,-1), (-4,-1)$, new foci as $(-1\pm\sqrt {13},-1)$. Step 3. To find the new asymptotes, keep the old slopes and use a new point $(-1,-1)$ to determine the y-intercepts. For the asymptote with positive slope, $y=\frac{2}{3}x+b$, plug-in $(-1,-1)$ to get $\frac{2}{3}(-1)+b=-1$ and $b=-\frac{1}{3}$ which gives the equation $y=\frac{2}{3}x-\frac{1}{3}$. For the other asymptote, $y=-\frac{2}{3}x+c$, plug-in $(-1,-1)$ to get $-\frac{2}{3}(-1)+c=-1$ and $c=-\frac{5}{3}$ which gives the equation $y=-\frac{2}{3}x-\frac{5}{3}$. (b) See graph.
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