What substitution is ideal here?

Calculus Level 3

Let A = 0 1 ( ln ( 4 3 x ) + 2 ln ( 1 + 3 x ) ) d x \displaystyle A = \int_0^1 \big( \ln(4-3^x) + 2\ln(1+ 3^x) \big ) \, dx .

Find e A e^A .


The answer is 16.

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1 solution

Naren Bhandari
Jul 6, 2020

A = 0 1 ( ln 4 + ln ( 1 3 x 4 ) + 2 ln ( 1 + 3 x ) ) d x A=\int_0^1\left(\ln 4 +\ln\left(1-\frac{3^x}{4}\right)+2\ln\left(1+3^x\right)\right)dx subbing 3 x 4 = y d x = d y y ln 3 \frac{3^x}{4}=y\implies dx =\frac{dy}{y \ln3 } yields A = ln 4 + 1 4 3 4 ( ln ( 1 y ) y + 2 ln ( 1 + 4 y ) y ) d y ln 3 = ln ( 4 ) 1 ln 3 [ Li 2 ( y ) + 2 Li 2 ( 4 y ) ] 1 4 3 4 = ln 4 1 ln 3 ( Li 2 ( 3 4 ) + Li 2 ( 1 4 ) 2 Li 2 ( 3 ) + 2 Li 2 ( 1 ) ) = ln 4 + ( η ( 2 ) ln ( 1 4 ) ln ( 3 4 ) ) 2 ( Li 2 ( 3 4 ) Li 2 ( 3 ) ) η ( 2 ) = ln 4 + 1 ln 3 ( ln ( 3 ) ln ( 4 ) ln 2 ( 4 ) + ln 2 ( 4 ) ) = ln ( 16 ) \begin{aligned} A &=\ln 4+\int_{\frac{1}{4}}^{\frac{3}{4}}\left( \frac{\ln(1-y)}{y}+\frac{2\ln(1+4y)}{y}\right)\frac{dy}{\ln3}\\&=\ln(4)-\frac{1}{\ln3}\left[\operatorname{Li}_{2}(y)+2\operatorname{Li}_{2}(-4y)\right]_{\frac{1}{4}}^{\frac{3}{4}}\\&=\ln4 -\frac{1}{\ln3}\left(\operatorname{Li}_{2}\left(\frac{3}{4}\right)+\operatorname{Li}_{2}\left(\frac{1}{4}\right)-2\operatorname{Li}_{2}(-3)+2\operatorname{Li}_{2}(-1)\right)\\&=\ln 4+ \left(\eta(2)-\ln\left(\frac{1}{4}\right)\ln\left(\frac{3}{4}\right)\right)-2\left(\operatorname {Li}_{2}\left(\frac{3}{4}\right)-\operatorname{Li}_{2}(-3)\right)-\eta(2)\\&= \ln4+\frac{1}{\ln3}\left( \ln(3)\ln(4)-\ln^2(4)+\ln^2(4)\right)\\&=\ln(16)\end{aligned} Thus e A = 16 e^A=16

Yay!!!!!!!!!!!

Pi Han Goh - 11 months, 1 week ago

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Haha,I have derived the next problem from this problem I will share it soon, sir.

Naren Bhandari - 11 months, 1 week ago

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I'm still weak in polylogarithms, so I didn't solve this "properly."

Thank you for your solution.

Pi Han Goh - 11 months, 1 week ago

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@Pi Han Goh Welcome sir. Please have a look at this .

Naren Bhandari - 11 months, 1 week ago

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