Can you solve it?

Calculus Level 4

L = lim x 0 ( ( 1 + x ) 1 / x e ) 1 sin x \large L = \lim_{x\to0} \left(\frac{(1+x)^{1/x}}{e}\right)^{\dfrac{1}{\sin{x}}}

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

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

1 2 \frac{1}{2} Limit Does not Exist None of these choices 1 2 -\frac{1}{2} 1 1

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2 solutions

Chew-Seong Cheong
Jun 12, 2016

L = lim x 0 ( ( 1 + x ) 1 x e ) 1 sin x = lim x 0 exp ( 1 sin x ( 1 x ln ( 1 + x ) 1 ) ) = lim x 0 exp ( 1 sin x ( 1 x ( x x 2 2 + x 3 3 . . . ) 1 ) ) = lim x 0 exp ( 1 sin x ( x 2 + x 2 3 x 3 4 . . . ) ) = lim x 0 exp ( 1 sin x x ( 1 2 + x 3 x 2 4 . . . ) ) = e 1 2 \begin{aligned} L & = \lim_{x \to 0} \left(\frac {(1 +x) ^\frac 1x} e\right) ^\frac 1{\sin x} \\ & = \lim_{x \to 0} \exp \left(\frac 1{\sin x} \left(\frac 1x\ln(1+x)-1\right) \right) \\ & = \lim_{x \to 0} \exp \left(\frac 1{\sin x} \left(\frac 1x\left (x-\frac {x^2} 2 +\frac {x^3}3-...\right) -1\right) \right) \\ & = \lim_{x \to 0} \exp \left(\frac 1{\sin x} \left(-\frac {x} 2 +\frac {x^2 }3-\frac {x^3}4...\right) \right) \\ & = \lim_{x \to 0} \exp \left(\frac 1{\frac {\sin x} x} \left(-\frac 1 2 +\frac {x}3-\frac {x^2 }4...\right) \right) \\ & = e^{-\frac 12} \end{aligned}

k = 1 2 \implies k = \boxed {-\frac 12}

Good...thanks sir

Mohit Gupta - 4 years, 3 months ago
Sam Bealing
Jun 12, 2016

In this solution, l o g log refers to the natural logarithm.

L = lim x 0 ( ( 1 + x ) 1 x e ) 1 sin x log ( L ) = lim x 0 ( log ( ( 1 + x ) 1 x e ) sin x ) = lim x 0 ( log ( 1 + x ) x log e sin x ) = lim x 0 ( log ( 1 + x ) x x sin x ) Use L’Hopital’s Rule as 0 0 = lim x 0 ( 1 1 + x 1 sin x + x cos x ) Use L’Hopital’s Rule again as 0 0 = lim x 0 ( 1 ( 1 + x ) 2 2 cos x x sin x ) = 1 2 log ( L ) = 1 2 L = e 1 2 k = 1 2 L= \lim_{x \to 0}{\left(\dfrac{(1+x)^{\frac{1}{x}}}{e} \right )^{\frac{1}{\sin{x}}}} \\ \begin{aligned} \log{\left(L \right)} &=\lim_{x \to 0} {\left(\dfrac{\log{\left (\frac{(1+x)^{\frac{1}{x}}}{e} \right )}}{\sin{x}} \right )} \\ &=\lim_{x \to 0} {\left(\dfrac{\frac{\log{(1+x)}}{x}-\log{e}}{\sin{x}} \right )} \\ &=\lim_{x \to 0}{\left(\dfrac{\log{(1+x)}-x}{x \sin{x}} \right )} \quad \quad \color{#3D99F6}{\text{Use L'Hopital's Rule as } \dfrac{0}{0}} \\ &=\lim_{x \to 0} {\left(\dfrac{\frac{1}{1+x}-1}{\sin{x}+x \cos{x}} \right )} \quad \quad \color{#3D99F6}{\text{Use L'Hopital's Rule again as } \dfrac{0}{0}} \\ &=\lim_{x \to 0} {\left(\dfrac{-\frac{1}{(1+x)^2}}{2 \cos{x}-x \sin{x}} \right )} \\ &=-\dfrac{1}{2} \end{aligned} \\ \log{\left(L \right)}=-\dfrac{1}{2} \implies L=e^{-\frac{1}{2}} \\ \implies \color{#20A900}{\boxed{\boxed{k=-\dfrac{1}{2}}}}

Moderator note:

Simple standard approach.

In such cases, I find that it's often most helpful to consider the Maclaurin expansion directly, since it's not clear how many applications of L'hopital are necessary.

@Sam Bealing , you should have noted that l o g log pertains to the natural logarithm l n ln because we have a different way of getting the derivatives of those with the form log b ( x ) \log_b (x) .

Kim Lehi Alterado - 5 years ago

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I have corrected that.

Sam Bealing - 4 years, 12 months ago

For calculus questions, log \log almost always refers to in base e e .

Calvin Lin Staff - 4 years, 12 months ago

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It is nice to learn something new (for me). Thank you, @Calvin Lin .

Kim Lehi Alterado - 4 years, 12 months ago

I did it by exact same method.

Akshay Yadav - 4 years, 12 months ago

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