Precalculus (6th Edition)

Published by Pearson
ISBN 10: 013421742X
ISBN 13: 978-0-13421-742-0

Chapter 10 - Analytic Geometry - 10.4 Summary of the Conic Sections - 10.4 Exercises - Page 997: 45

Answer

$$e = \frac{1}{3}$$

Work Step by Step

$$\eqalign{ & {\text{From the graph we can see:}} \cr & {\text{Focus }}\left( {3,0} \right){\text{ and the point }}\left( { - 3,8} \right) \cr & {\text{The equation of the ellipse is }}\frac{{{x^2}}}{{{a^2}}} + \frac{{{y^2}}}{{{b^2}}} = 1 \cr & {\text{Foci}}\left( { \pm c,0} \right) \to c = 3 \cr & {c^2} = {a^2} - {b^2} \cr & {a^2} - {b^2} = 9 \cr & {b^2} = {a^2} - 9 \cr & \cr & {\text{Substitute the point }}\left( { - 3,8} \right){\text{ into }}\frac{{{x^2}}}{{{a^2}}} + \frac{{{y^2}}}{{{b^2}}} = 1 \cr & \frac{9}{{{a^2}}} + \frac{{64}}{{{b^2}}} = 1 \cr & {\text{Substitute }}{b^2} = {a^2} - 9 \cr & \frac{9}{{{a^2}}} + \frac{{64}}{{{a^2} - 9}} = 1 \cr & {\text{Solving for }}{a^2}{\text{ we obtain}} \cr & {a^2} = 81 \cr & a = 9 \cr & {\text{Find the eccentricity}} \cr & e = \frac{3}{9} \cr & e = \frac{1}{3} \cr} $$
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