How many integrals ?

Calculus Level pending

Evaluate the integral I = 0 1 ( x x 3 ) ln ( x + x 3 ) x + x 3 d x I=\displaystyle \int_0^1\frac{(x-\sqrt[3]{x})\ln(x+\sqrt[3]{x})}{x+\sqrt[3]{x}}dx submit your answer as 1 0 9 I \lfloor10^9I\rfloor .

This problem, taken from Romanian Mathematical Magazine , proposed by Sir Srinivasa Raghava .


The original proposed problem supposed to evaluate the integral is closed form.


The answer is 79860950.

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1 solution

Naren Bhandari
May 9, 2020

One of the approach which I wish to share. 0 1 ( x x 3 ) ln ( x + x 3 ) x + x 3 d x = x = u 3 3 0 1 u ( u 3 u ) ln ( u 3 + u ) u 2 + 1 d u \int_{0}^1\frac{(x-\sqrt[3]{x})\ln(x+\sqrt[3]{x})}{x+\sqrt[3]{x}}dx \overbrace{=}^{x=u^3}3\int_0^1\frac{u(u^3-u)\ln(u^3+u)}{u^2+1}du the latter integral we write as 3 1 I = 0 1 ( u 4 ln ( u 3 + u ) u 2 + 1 u 2 ln ( u 3 + u ) u 2 + 1 ) 3^{-1}I=\int_0^1\left(\frac{u^4\ln(u^3+u)}{u^2+1}-\frac{u^2\ln(u^3+u)}{u^2+1}\right) let u = tan y u=\tan y then the integral becomes 0 π 4 tan 4 y ln ( tan y sec 2 y ) tan 2 y ln ( tan y sec 2 y ) d y \int_0^{\frac{\pi}{4}}\tan^4y\ln(\tan y \sec^2y)-\tan^2y\ln(\tan y \sec^2y)dy 0 π 4 ( tan 4 y tan 2 y ) ln ( tan y ) d y I 1 + 0 π 4 ( tan 4 y tan 2 y ) ln ( sec 2 y ) d y I 2 \underbrace{\int_0^{\frac{\pi}{4}}\left(\tan^4y-\tan^2y\right)\ln(\tan y)dy}_{I_1}+\underbrace {\int_0^{\frac{\pi}{4}}(\tan^4y-\tan^2 y)\ln(\sec^2 y)dy}_{I_2} By using the reduction formula of I ( n ) = tan n y d y I(n)=\displaystyle \int\tan^n ydy for n = 4 n=4 we deduce that I ( 4 ) = tan 4 y = y + 1 3 sec 2 y tan y 4 3 + C I(4)=\int \tan^4 y=y+\frac{1}{3} \sec^2y \tan y-\frac{4}{3}+C and n = 2 n=2 is trivial to see I ( 2 ) = tan 2 y = tan y y + C \displaystyle I(2)=\int\tan^2 y=\tan y-y+C Now we calculate I 1 , I 2 I_1,I_2 using integration by parts I 1 = ln ( tan y ) 0 π 4 ( tan 4 y tan 2 y ) d y 0 0 π 4 sec 2 y tan y ( I ( 4 ) I ( 2 ) ) d y I_1=\underbrace{\ln(\tan y)\int_0^{\frac{\pi}{4}}(\tan^4y-\tan^2 y)dy}_{0}-\int_0^{\frac{\pi}{4}}\frac{\sec^2y}{\tan y}(I(4)-I(2))dy Further I 1 = 0 π 4 sec 2 y tan y ( 2 y + tan y 3 ( sec 2 y 7 ) ) d y I_1=\int_0^{\frac{\pi}{4}}\frac{\sec^2 y}{\tan y}\left(2y+\frac{\tan y}{3}\left(\sec^2y-7\right)\right)dy = 0 π 4 2 ( y tan y + y cot y ) d y I 3 + 1 3 0 π 4 sec 2 y ( sec 2 y 7 ) d y I 4 =\underbrace {\int_0^{\frac{\pi}{4}} 2\left(y\tan y +y\cot y\right)dy}_{I_3}+\underbrace {\frac{1}{3}\int_{0}^{\frac{\pi}{4}} \sec^2y(\sec^2 y-7)dy}_{I_4} Here I 3 = y 0 π 4 ( tan y + cot y ) 0 0 π 4 ( ( tan y + cot y ) d y ) d y I_3=\overbrace{y\int_0^{\frac{\pi}{4}}(\tan y+\cot y)}^{0}-\int_{0}^{\frac{\pi}{4}}\left(\int(\tan y +\cot y)dy\right)dy = 0 π 4 ln ( tan y ) d y = tan y = t 0 1 ln t 1 + t 2 d t = n = 1 ( 1 ) n ( 2 n 1 ) 2 = G I 3 = 2 G =-\int_0^{\frac{\pi}{4}}\ln(\tan y)dy \overbrace{=}^{\tan y=t}- \int_0^1\frac{\ln t}{1+t^2}dt =-\sum_{n=1}^{\infty}\frac{(-1)^{n}}{(2n-1)^2}=G\Rightarrow I_3=-2G Further we evaluate 3 I 4 = 0 π 4 sec 2 y ( sec 2 y 7 ) d y = 0 π 4 ( sec 2 y tan 2 y 6 tan 2 y ) 3I_4=\int_0^{\frac{\pi}{4}} \sec^2y(\sec^2y-7)dy=\int_0^{\frac{\pi}{4}}(\sec^2y\tan^2y-6\tan^2y) = [ tan 3 y 3 6 tan y ] 0 π 4 = 17 3 I 4 = 17 9 =\left[\frac{\tan^3y}{3}-6\tan y\right]_0^{\frac{\pi}{4}}= -\frac{17}{3}\Rightarrow I_4=\frac{17}{9} and hence I 1 = 0 π 4 ( tan 4 y tan 2 y ) ln ( tan y ) d y = 17 9 2 G I_1=\int_0^{\frac{\pi}{4}}\left(\tan^4y-\tan^2y\right)\ln(\tan y)dy=\frac{17}{9}-2G Now we evaluate I 2 I_2 0 π 4 ( tan 4 y tan 2 y ) ln ( sec 2 y ) d y = 2 0 π 4 ( tan 4 y tan 2 y ) ln ( cos y ) d y \int_0^{\frac{\pi}{4}}(\tan^4y-\tan^2 y)\ln(\sec^2 y)dy=-2\int_0^{\frac{\pi}{4}}(\tan^4y-\tan^2 y)\ln(\cos y)dy = I B P 2 ln ( cos y ) 0 π 4 ( tan 4 tan 2 y ) d y + 2 0 π 4 ( ( tan y ) ( tan 4 y tan 2 y ) d y ) d y \overbrace{=}^{IBP}-2\ln(\cos y)\int_0^{\frac{\pi}{4}}(\tan^4-\tan^2 y)dy+2\int_0^{\frac{\pi}{4}}\left( (-\tan y)\int(\tan^4 y-\tan^2 y)dy\right)dy = 5 3 ln 2 + π 2 ln 2 2 0 π 4 tan y ( 2 y + 1 3 sec 2 y tan y 7 3 tan y ) d y =-\frac{5}{3}\ln2+\frac{\pi}{2}\ln2-2\int_0^{\frac{\pi}{4}}\tan y\left(2y+\frac{1}{3}\sec^2 y\tan y-\frac{7}{3}\tan y\right)dy Further 0 π 4 ( 4 y tan y 2 3 sec 2 y tan 2 y + 14 3 tan 2 y ) d y = 4 0 π 4 y tan y + 40 9 7 π 6 \int_0^{\frac{\pi}{4}}\left(-4y\tan y-\frac{2}{3}\sec^2y\tan^2y+\frac{14}{3}\tan^2y\right)dy=-4\int_0^{\frac{\pi}{4}}y\tan y +\frac{40}{9}-\frac{7\pi}{6}

= 4 y 0 π 4 tan y + 4 0 π 4 ( tan y d y ) d y = π 2 ln 2 4 0 π 4 ln ( cos y ) d y =-4y\int_0^{\frac{\pi}{4}}\tan y +4\int_0^{\frac{\pi}{4}}\left(\int\tan y dy\right)dy=-\frac{\pi}{2} \ln 2-4\int_0^{\frac{\pi}{4}}\ln(\cos y)dy Let J 1 = 0 π 4 ln ( cos y ) d y , J 2 = 0 π 4 ln ( sin y ) d y \displaystyle J_1 =\int_0^{\frac{\pi}{4}}\ln(\cos y)dy \; ,J_2=\int_0^{\frac{\pi}{4}}\ln(\sin y) dy Note that J 1 + J 2 J_1+J_2 = 0 π 4 ln ( 1 2 sin 2 y ) d y = 2 y = t 1 2 0 π 2 ln ( sin t ) d t π 4 ln 2 = π 4 ln 2 π 4 ln 2 = π 2 ln 2 =\int_0^{\frac{\pi}{4}}\ln\left(\frac{1}{2}\sin2 y\right)dy\overbrace{=}^{2y=t}\frac{1}{2}\int_0^{\frac{\pi}{2}}\ln(\sin t)dt -\frac{\pi}{4}\ln 2 =-\frac{\pi}{4}\ln 2 -\frac{\pi}{4}\ln 2 =-\frac{\pi}{2}\ln 2 Also J 2 J 1 = 0 π 4 ln ( tan y ) d y = I 1 G J_2-J_1=\int_0^{\frac{\pi}{4}}\ln(\tan y)dy\overbrace{=}^{ I_1} -G So = π 2 ln 2 4 0 π 4 ln ( cos y ) d y = π 2 ln 2 4 ( G 2 π 2 ln 2 ) = π 2 ln 2 2 G \displaystyle=-\frac{\pi}{2}\ln 2-4\int_0^{\frac{\pi}{4}}\ln (\cos y)dy= -\frac{\pi}{2}\ln 2 -4\left(\frac{G}{2}-\frac{\pi}{2}\ln2\right)=\frac{\pi}{2}\ln 2 -2G Making the finally answer 3 ( I 1 + I 2 ) 3(I_1+I_2) = 3 ( 40 + 17 9 4 G + π ln 2 5 3 ln 2 7 π 3 ) = 19 12 G + 3 π ln 2 5 ln ( 2 ) 7 π 2 0.07986095042 \displaystyle= 3\left(\frac{40+17}{9}-4G+\pi\ln 2-\frac{5}{3}\ln 2 -\frac{7\pi}{3}\right)=19-12G+3\pi\ln2 -5\ln(2) -\frac{7\pi}{2}\approx 0.07986095042\cdots


The solution I had is quite lengthy and to speed up the calculation and reduce the work I even tried up with generating function for harmonic numbers which seems to be tedious again with calculations. So im looking for any short solution/s. I will be glad to know if anyone wish to share their approach.

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