Something to do with e?

Calculus Level 5

It is known that lim x 0 ( 1 + x ) 1 / x e x \lim_{x\to0} \dfrac{(1+x)^{1/x} - e}{x} exists, and let it be A A .

Find the following limit, if it exists, otherwise key in 9999. lim x 0 ( 1 + x ) 1 / x e x A x \lim_{x\to0} \dfrac{\dfrac{(1+x)^{1/x} - e}{x}-A}{x}


Inspiration


The answer is 1.24588.

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1 solution

Dwaipayan Shikari
Jan 30, 2021

lim x 0 ( 1 + x ) 1 x e x = A lim x 0 log ( 1 + x ) x = log ( A x + e ) \lim_{x\rightarrow{0}} \dfrac{(1+x)^{\frac{1}{x}}-e}{x} =A \implies \lim_{x \rightarrow{0}} \dfrac{\log(1+x)}{x}= \log(Ax+e) Maclaurin series for log ( 1 + x ) = x x 2 2 + x 3 3 \log(1+x)= x-\dfrac{x^2}{2}+\dfrac{x^3}{3}-\cdots lim x 0 x x 2 2 + x = log ( e ) + log ( A x e + 1 ) \lim_{x\rightarrow {0}} \dfrac{x-\frac{x^2}{2}+\cdots}{x}=\log(e)+\log(\frac{Ax}{e}+1) Using lim x 0 log ( 1 + x ) = x \lim_{x\rightarrow{0}}\log(1+x)=x We get 1 x 2 = 1 + A x e A = e 2 1-\dfrac{x}{2} = 1+\dfrac{Ax}{e} \implies A=-\dfrac{e}{2} Using Maclaurin series of e log ( 1 + x ) x e^{\frac{\log(1+x)}{x}} e log ( 1 + x ) x = lim x 0 ( 1 + x ) 1 x = e 1 x 2 + x 2 3 + e^{\frac{\log(1+x)}{x}}=\lim_{x\rightarrow{0}}(1+x)^{\frac{1}{x}}= e^{1-\frac{x}{2}+\frac{x^2}{3}+\cdots} lim x 0 ( 1 + x ) 1 x = e e x 2 + 11 e x 2 24 + \lim_{x\rightarrow{0}} (1+x)^{\frac{1}{x}}= e-\dfrac{ex}{2}+\dfrac{11ex^2}{24}+\cdots lim x 0 ( 1 + x ) 1 x e x = e 2 + 11 e x 24 \lim_{x\rightarrow{0}} {\dfrac{(1+x)^{\frac{1}{x}}-e}{x}}=-\dfrac{e}{2}+\dfrac{11ex}{24} lim x 0 ( 1 + x ) 1 x e x A x = 11 e x 24 x = 11 e 24 \lim_{x\rightarrow{0}} \dfrac{{\dfrac{(1+x)^{\frac{1}{x}}-e}{x}-A}{}}{x}=\dfrac{11ex}{24x}= \dfrac{11e}{24}

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