Calculus with Applications (10th Edition)

Published by Pearson
ISBN 10: 0321749006
ISBN 13: 978-0-32174-900-0

Chapter 12 - Sequences and Series - 12.7 L'Hospital's Rule - 12.7 Exercises - Page 659: 31

Answer

$$5$$

Work Step by Step

$$\eqalign{ & \mathop {\lim }\limits_{x \to 0} \frac{{\left( {5 + x} \right)\ln \left( {x + 1} \right)}}{{{e^x} - 1}} \cr & {\text{Verify if the conditions of l'Hospital's rule are satisfied}}{\text{. Here}} \cr & {\text{evaluate the limit substituting }}0{\text{ for }}x{\text{ into the numerator and }} \cr & {\text{denominator}}{\text{.}} \cr & \mathop {\lim }\limits_{x \to 0} \left( {5 + x} \right)\ln \left( {x + 1} \right) = \left( {5 + 0} \right)\ln \left( {0 + 1} \right) = \left( 5 \right)\left( 0 \right) = 0 \cr & \mathop {\lim }\limits_{x \to 0} \left( {{e^x} - 1} \right) = 0 = 1 - 1 = 0 \cr & \cr & {\text{Since the limits of both numerator and denominator are 0}}{\text{, }} \cr & {\text{l'Hospital's rule applies}}{\text{. Differentiating the numerator and}} \cr & {\text{denominator}}{\text{:}} \cr & {\text{for }}\left( {5 + x} \right)\ln \left( {x + 1} \right) \to {D_x}\left( {\left( {5 + x} \right)\ln \left( {x + 1} \right)} \right) \cr & {\text{By the product rule}} \cr & = \left( {5 + x} \right){D_x}\left( {\ln \left( {x + 1} \right)} \right) + \ln \left( {x + 1} \right){D_x}\left( {\left( {5 + x} \right)} \right) \cr & = \left( {5 + x} \right)\left( {\frac{1}{{x + 1}}} \right) + \ln \left( {x + 1} \right)\left( 1 \right) \cr & = \frac{{x + 5}}{{x + 1}} + \ln \left( {x + 1} \right) \cr & \cr & {\text{for }}{e^x} - 1 \to {D_x}\left( {{e^x} - 1} \right) = {e^x} \cr & {\text{then}} \cr & \mathop {\lim }\limits_{x \to 0} \frac{{\left( {5 + x} \right)\ln \left( {x + 1} \right)}}{{{e^x} - 1}} = \mathop {\lim }\limits_{x \to 0} \frac{{\frac{{x + 5}}{{x + 1}} + \ln \left( {x + 1} \right)}}{{{e^x}}} \cr & {\text{Find the limit of the quotient of the derivatives}} \cr & = \frac{{\frac{{0 + 5}}{{0 + 1}} + \ln \left( {0 + 1} \right)}}{{{e^0}}} \cr & = \frac{{5 + 0}}{1} \cr & = 5 \cr & {\text{By l'Hospital' rule}}{\text{, this result is the desired limit:}} \cr & \mathop {\lim }\limits_{x \to 0} \frac{{\left( {5 + x} \right)\ln \left( {x + 1} \right)}}{{{e^x} - 1}} = 5 \cr} $$
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