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Calculus Level 3

Find

lim x 1 x 1 x 1 . \displaystyle \Large \lim _{ x\to 1 }{ x^{ \frac { 1 }{ x-1 } } } .


The answer is 2.71828182.

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3 solutions

L = lim x 1 x 1 x 1 It is 1 form, use lim x a ( f ( x ) g ( x ) ) h ( x ) = e lim x a h ( x ) ( f ( x ) g ( x ) 1 ) = e lim x 1 1 x 1 ( x 1 ) = e 2.71828182 \begin{aligned} L & = \lim_{x \to 1} x^{\frac 1{x-1}} & \small \color{#3D99F6} \text{It is }1^\infty \text{ form, use } \lim_{x \to a} \left(\frac {f(x)}{g(x)} \right)^{h(x)} = e^{\lim_{x \to a} h(x) \left(\frac {f(x)}{g(x)} - 1 \right)} \\ & = e^{\lim_{x \to 1} \frac 1{x-1} (x-1)} \\ & = e \approx \boxed{2.71828182} \end{aligned}

Nowras Otmen
Dec 7, 2016

Let L = lim x 1 x 1 x 1 L=\lim_{x\rightarrow 1} x^{\frac{1}{x-1}} . We can rewrite L using the exponential in order to make it simpler in the following way

L = lim x 1 x 1 x 1 = lim x 1 exp ( ln x x 1 ) L=\lim_{x\rightarrow 1} x^{\frac{1}{x-1}}=\lim_{x\rightarrow 1} \exp\left(\dfrac{\ln x}{x-1}\right)

By doing this, we get to the indeterminate form 0 / 0 0/0 and it is a simple application of L'Hopital's

L = lim x 1 exp ( ln x x 1 ) = lim x 1 exp ( 1 x ) = exp ( 1 ) = e . L=\lim_{x\rightarrow 1} \exp\left(\dfrac{\ln x}{x-1}\right)=\lim_{x\rightarrow 1} \exp\left(\dfrac{1}{x}\right)=\exp(1)=e.

梦 叶
Dec 7, 2016

lim x to infinity (1-x)^(1/x) = e

How does this relate to the given problem?

Michael Huang - 4 years, 6 months ago

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the question is equivalent to (1+x-1)^(1/(x-1))=(1-(1-x))^(1/(x-1)), then apply the formula

梦 叶 - 4 years, 6 months ago

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Interesting. :)

Michael Huang - 4 years, 6 months ago

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