Lazy Limits 1!

Calculus Level 3

lim x 0 + ( csc x ) 1 / ln x = ? \large \lim_{x\to0^+} ( \csc x)^{1/\ln x} = \, ?

e 2 e^{-2} e 1 e^{-1} e 1 e^1 e 2 e^2

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2 solutions

L = lim x 0 + ( csc x ) 1 ln x = exp [ ln ( lim x 0 + ( csc x ) 1 ln x ) ] exp ( z ) = e z = exp [ lim x 0 + ( ln ( sin x ) ln x ) ] As it is a / case, L’H o ˆ pital’s rule applies. = exp [ lim x 0 + ( cos x sin x 1 x ) ] Differentiating up and down w.r.t. x = exp [ lim x 0 + ( cos x sin x x ) ] x 0 sin x x 1 = exp [ 1 1 ] = e 1 \begin{aligned} \mathscr L & = \lim_{x \to 0^+} (\csc x)^\frac 1{\ln x} \\ & = \color{#3D99F6}{\exp} \left[ \ln \left( \lim_{x \to 0^+} (\csc x)^\frac 1{\ln x} \right) \right] \quad \quad \small \color{#3D99F6}{\exp (z) = e^z} \\ & = \exp \left[ \lim_{x \to 0^+} \left(\color{#3D99F6}{ - \frac {\ln (\sin x)}{\ln x}} \right) \right] \quad \quad \small \color{#3D99F6}{\text{As it is a }\infty / \infty \text{ case, L'Hôpital's rule applies.}} \\ & = \exp \left[ \lim_{x \to 0^+} \left( \color{#3D99F6}{- \frac {\frac {\cos x}{\sin x}}{\frac 1x}} \right) \right] \quad \quad \small \color{#3D99F6}{\text{Differentiating up and down w.r.t. } x} \\ & = \exp \left[ \lim_{x \to 0^+} \left(- \frac {\cos x}{\color{#3D99F6}{\frac {\sin x}x}} \right) \right] \quad \quad \small \color{#3D99F6}{x \to 0 \implies \frac {\sin x}x \to 1} \\ & = \exp \left[ - \frac 11 \right] = \boxed{e^{-1}} \end{aligned}

lim x 0 + ( csc x ) 1 ln x = e ln lim x 0 + ( csc x ) 1 ln x = \large \displaystyle \lim_{x\to 0^{+}} (\csc x)^{\frac{1}{\ln x}} = \displaystyle e^{\ln {\lim_{x\to 0^{+}} (\csc x)^{\frac{1}{\ln x}}}} = due to function ln x \ln x is a continuous function = e lim x 0 + 1 ln x ln csc x = \large \displaystyle = e^{\lim_{x\to 0^{+}} \frac{1}{\ln x}\cdot \ln \csc x} = Applying L'Hopital's Rule = e lim x 0 + x ( cot x csc x ) csc x = e lim x 0 + x tan x = e 1 \large \displaystyle = e^{\lim_{x\to 0^{+}} \frac{x \cdot (- \cot x \cdot \csc x) }{\csc x}} = e^{\lim_{x\to 0^{+}} \frac{-x}{\tan x}} = e^{-1}

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