L'Hospital's Rule?

Calculus Level 5

L = lim x 0 ( 1 + x ) 1 / x e + e 2 x x 2 \large L = \lim_{x\to0} \dfrac{(1+x)^{1/x} - e + \frac e2 x }{x^2}

If L = k e L = ke , where k k is a real number , find the value of k k .

Give your answer to 3 decimal places.

If you think that the limit does not exist, enter 0.666 as your answer.

Clarification : e e denotes Euler's number , e 2.71828 e \approx 2.71828 .


The answer is 0.4583.

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1 solution

Sabhrant Sachan
May 30, 2016

Relevant wiki: Maclaurin Series

we know the expansion of ln ( x + 1 ) , it is \text{we know the expansion of } \ln{(x+1)} \text{, it is }

ln ( x + 1 ) = x x 2 2 + x 3 3 + 1 x ln ( x + 1 ) = 1 x 2 + x 2 3 + e 1 x ln ( x + 1 ) = e 1 x 2 + x 2 3 + ( x + 1 ) 1 x = e e x 2 + x 2 3 + ( x + 1 ) 1 x = e ( 1 1 2 x + 11 24 x 2 + ) ( x + 1 ) 1 x = e e 2 x + 11 e 24 x 2 + Plugging this value in our Limit L = lim x 0 e e 2 x + 11 e 24 x 2 + e + e 2 x x 2 11 e 24 L = 0.4583 \ln{(x+1)} = x-\dfrac{x^2}{2}+\dfrac{x^3}{3}+\cdots \\ \dfrac{1}{x}\ln{(x+1)} = 1-\dfrac{x}{2}+\dfrac{x^2}{3}+\cdots \\ e^{\frac{1}{x}\ln{(x+1)}}=e^{1-\frac{x}{2}+\frac{x^2}{3}+\cdots} \\ (x+1)^{\frac1{x}}=e\cdot e^{-\frac{x}{2}+\frac{x^2}{3}+\cdots} \\ (x+1)^{\frac1{x}}=e\cdot(1-\dfrac{1}{2}x+\dfrac{11}{24}x^2+\cdots) \\ (x+1)^{\frac1{x}}=e-\dfrac{e}{2}x+\dfrac{11e}{24}x^2+\cdots \\ \text{Plugging this value in our Limit } \\ L = \displaystyle\lim_{x \to 0} \dfrac{ \cancel{e}-\cancel{\dfrac{e}{2}x}+\dfrac{11e}{24}x^2+\cdots -\cancel{e}+\cancel{\dfrac{e}{2}x}}{x^2} \implies \boxed{\dfrac{11e}{24}} \\ L = 0.4583

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