Calculus, 10th Edition (Anton)

Published by Wiley
ISBN 10: 0-47064-772-8
ISBN 13: 978-0-47064-772-1

Chapter 6 - Exponential, Logarithmic, And Inverse Trigonometric Functions - Chapter 6 Review Exercises - Page 485: 56

Answer

$$ - \frac{1}{e}$$

Work Step by Step

$$\eqalign{ & \mathop {\lim }\limits_{x \to e} \frac{{1 - \ln x}}{{\left( {x - e} \right)\ln x}} \cr & = \mathop {\lim }\limits_{x \to e} \frac{{\frac{1}{{\ln x}} - \frac{{\ln x}}{{\ln x}}}}{{\frac{{\left( {x - e} \right)\ln x}}{{\ln x}}}} \cr & = \mathop {\lim }\limits_{x \to e} \frac{{\frac{1}{{\ln x}} - 1}}{{x - e}} = \mathop {\lim }\limits_{x \to e} \frac{{\frac{1}{{\ln x}} - \frac{1}{{\ln \left( e \right)}}}}{{x - e}} \cr & {\text{From the definition of the derivative }} \cr & f'\left( c \right) = \mathop {\lim }\limits_{x \to c} \frac{{f\left( x \right) - f\left( c \right)}}{{x - c}} \cr & \underbrace {\mathop {\lim }\limits_{x \to e} \frac{{\frac{1}{{\ln x}} - \frac{1}{{\ln \left( e \right)}}}}{{x - e}}}_{f'\left( c \right) = \mathop {\lim }\limits_{x \to c} \frac{{f\left( x \right) - f\left( c \right)}}{{x - c}}} \Rightarrow f\left( x \right) = \frac{1}{{\ln x}},{\text{ }}a = 3 \cr & {\text{Then }} \cr & f'\left( x \right) = \frac{d}{{dx}}\left[ {\frac{1}{{\ln x}}} \right] = - {\left( {\ln x} \right)^{ - 2}}\left( {\frac{1}{x}} \right) \cr & f'\left( x \right) = - \frac{1}{{x{{\left( {\ln x} \right)}^2}}} \cr & {\text{Therefore,}} \cr & \mathop {\lim }\limits_{x \to e} \frac{{1 - \ln x}}{{\left( {x - e} \right)\ln x}} = f'\left( e \right) \cr & f'\left( e \right) = - \frac{1}{{e{{\left( {\ln e} \right)}^2}}} = - \frac{1}{e} \cr & \mathop {\lim }\limits_{x \to e} \frac{{1 - \ln x}}{{\left( {x - e} \right)\ln x}} = - \frac{1}{e} \cr} $$
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