All the logarithms

Calculus Level 5

f ( x ) = ln ( 1 x 2 ) + ln ( 1 x 8 ) + ln ( 1 x 24 ) + + ln ( 1 x n 2 n ) + f\left( x \right) =\ln { \left( \frac { 1 }{ \sqrt [ 2 ]{ \left| x \right| } } \right) } +\ln { \left( \frac { 1 }{ \sqrt [ 8 ]{ \left| x \right| } } \right) } +\ln { \left( \frac { 1 }{ \sqrt [ 24 ]{ \left| x \right| } } \right) } +\dots +\ln { \left( \frac { 1 }{ \sqrt [ n{ 2 }^{ n } ]{ \left| x \right| } } \right) } + \ldots

Find the area bounded by f ( x ) f\left( x \right) and the x x -axis. If this area can be expressed in the form ln A \ln { A } , find A A .


The answer is 4.

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1 solution

Chew-Seong Cheong
Nov 12, 2017

f ( x ) = n = 1 ln ( 1 x n 2 n ) = n = 1 ln x n 2 n By Maclaurin series = ln ( 1 1 2 ) ln x = ln 2 ln x \begin{aligned} f(x) & = \sum_{n=1}^\infty \ln \left(\frac 1{\sqrt[n2^n]{|x|}}\right) \\ & = \sum_{n=1}^\infty - \frac {\ln |x|}{n2^n} & \small \color{#3D99F6} \text{By Maclaurin series} \\ & = \ln \left(1-\frac 12\right)\ln|x| \\ & = - \ln 2 \ln|x| \end{aligned}

The area bounded by f ( x ) f(x) and the x x -axis is given by:

I = 1 1 f ( x ) d x = ln 2 1 1 ln x d x Since the integrand is even = 2 ln 2 0 1 ln x d x = 2 ln 2 ( x ln x x ) 0 1 = 2 ln 2 ( 1 ln 1 1 lim x 0 x ln x + 0 ) By L’H o ˆ pital’s rule (see note) = 2 ln 2 ( 0 1 0 + 0 ) = 2 ln 2 = ln 4 \begin{aligned} I & = \int_{-1}^1 f(x) \ dx \\ & = - \ln 2 \int_{-1}^1 \ln |x| \ dx & \small \color{#3D99F6} \text{Since the integrand is even} \\ & = - 2 \ln 2 \int_0^1 \ln |x| \ dx \\ & = - 2 \ln 2 (x\ln x - x)\bigg|_0^1 \\ & = - 2 \ln 2 \left(1\ln 1 - 1 - {\color{#3D99F6}\lim_{x \to 0} x\ln x} + 0\right) & \small \color{#3D99F6} \text{By L'Hôpital's rule (see note)} \\ & = - 2 \ln 2 \left(0 - 1 - {\color{#3D99F6}0} + 0\right) \\ & = 2\ln 2 \\ & = \ln 4 \end{aligned}

A = 4 \implies A = \boxed{4}


Note:

lim x 0 x ln x = lim x 0 ln x 1 x A / case, L’H o ˆ pital’s rule applies. = lim x 0 1 x 1 x 2 Differentiate up and down w.r.t. x = lim x 0 1 x × x 2 = 0 \begin{aligned} \lim_{x \to 0} x\ln x & = \lim_{x \to 0} \frac {\ln x}{\frac 1x} & \small \color{#3D99F6} \text{A }\infty/\infty \text{ case, L'Hôpital's rule applies.} \\ & = \lim_{x \to 0} \frac {\frac 1x}{-\frac 1{x^2}} & \small \color{#3D99F6} \text{Differentiate up and down w.r.t. }x \\ & = \lim_{x \to 0} - \frac 1x\times x^2 \\ & = 0 \end{aligned}

same way :)

Arghyadeep Chatterjee - 2 years, 4 months ago

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