"Limit"ed Edition

Calculus Level 4

lim x 1 ( x x 1 1 ln x ) = ? \large \lim_{x\to1 } \left ( \dfrac x{x-1} - \dfrac1{\ln x} \right) =\, ?


The answer is 0.5.

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1 solution

Zach Abueg
Feb 3, 2017

lim x 1 x x 1 1 ln x = \displaystyle \lim_{x \to\ 1} \dfrac {x}{x - 1} - \dfrac {1}{\ln x} = \infty - \infty

Apply L'hopital's Rule:

lim x 1 x ln x x + 1 ( x 1 ) ( ln x ) = 0 0 \displaystyle \lim_{x \to\ 1} \dfrac {x \ln x - x + 1}{(x - 1)(\ln x)} = \dfrac 00

Apply L'hopital's Rule again:

lim x 1 1 + ln x 1 x 1 x + ln x = 0 0 \displaystyle \lim_{x \to\ 1} \dfrac{1 + \ln x - 1}{\dfrac {x - 1}{x} + \ln x} = \dfrac 00

And one final time:

lim x 1 1 x 1 x 2 + 1 x = 1 2 \displaystyle \lim_{x \to\ 1} \dfrac{\dfrac1x}{\dfrac{1}{x^2} + \dfrac 1x} = \dfrac 12

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