Calculus: Early Transcendentals 8th Edition

Published by Cengage Learning
ISBN 10: 1285741552
ISBN 13: 978-1-28574-155-0

Chapter 4 - Section 4.4 - Indeterminate Forms and l''Hospital''s Rule - 4.4 Exercises - Page 312: 68

Answer

$\lim\limits_{x \to \infty}(\frac{2x-3}{2x+5})^{2x+1} = \frac{1}{e^{8}}$

Work Step by Step

Let $~~y = \lim\limits_{x \to \infty}(\frac{2x-3}{2x+5})^{2x+1}$ $ln~y = \lim\limits_{x \to \infty}ln (\frac{2x-3}{2x+5})^{2x+1}$ $ln~y = \lim\limits_{x \to \infty}(2x+1)~ln (\frac{2x-3}{2x+5})$ $ln~y = \lim\limits_{x \to \infty}\frac{ln (\frac{2x-3}{2x+5}~)}{(2x+1)^{-1}} = \frac{0}{0}$ We can use L'Hospital's Rule: $ln~y = \lim\limits_{x \to \infty}\frac{\frac{2x+5}{2x-3}~\cdot ~\frac{2(2x+5)-2(2x-3)}{(2x+5)^2}}{-2(2x+1)^{-2}}$ $ln~y = \lim\limits_{x \to \infty}\frac{\frac{16}{(2x-3)(2x+5)}}{-2(2x+1)^{-2}}$ $ln~y = \lim\limits_{x \to \infty}\frac{-8(2x+1)^2}{(2x-3)(2x+5)}$ $ln~y = \lim\limits_{x \to \infty}\frac{-8(4x^2+4x+1)}{4x^2+4x-15}$ $ln~y = \lim\limits_{x \to \infty}\frac{-32x^2-32x-8}{4x^2+4x-15}$ $ln~y = \lim\limits_{x \to \infty}\frac{-32-32/x-8/x^2}{4+4/x-15/x^2}$ $ln~y = -\frac{32}{4}$ $ln~y = -8$ $y = e^{-8}$ $y = \frac{1}{e^{8}}$
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