Inspired by Kishore S Shenoy

Calculus Level 5

0 1 ln ( 1 + x x 2 ) x d x \large \int_0^1 \dfrac{\ln(1+x-x^2)}x \, dx

If the value of the integral above is equal to 2 ( ln A ) 2 2 (\ln A)^2 , find the positive value of A ( A 3 3 ) A(A^3- 3) .


Inspiration .


The answer is 2.

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2 solutions

Mark Hennings
Jan 17, 2016

We start with a bit of variable manipulation: I = 0 1 ln ( 1 + x x 2 ) x d x = ( 0 1 2 + 1 2 1 ) ln ( 1 + x ( 1 x ) ) x d x = 0 1 2 ln ( 1 + x ( 1 x ) ) x d x + 0 1 2 ln ( 1 + x ( 1 x ) ) 1 x d x = 0 1 2 ln ( 1 + x ( 1 x ) ) x ( 1 x ) d x = 0 1 2 ( m = 0 ( 1 ) m m + 1 x m ( 1 x ) m ) d x = m = 0 ( 1 ) m m + 1 0 1 2 x m ( 1 x ) m d x = m = 0 ( 1 ) m m + 1 0 1 4 π sin 2 m θ cos 2 m θ 2 sin θ cos θ d θ = 2 m = 0 ( 1 ) m m + 1 0 1 4 π sin 2 m + 1 θ cos 2 m + 1 θ d θ = 2 m = 0 ( 1 ) m ( m + 1 ) 2 2 m + 1 0 1 4 π sin 2 m + 1 2 θ d θ = m = 0 ( 1 ) m ( m + 1 ) 2 2 m + 1 0 1 2 π sin 2 m + 1 θ d θ = m = 0 ( 1 ) m ( m + 1 ) 2 2 m + 1 ( 2 m ) ! ! ( 2 m + 1 ) ! ! \begin{array}{rcl} \displaystyle I & = &\displaystyle \int_0^1 \frac{\ln(1 + x - x^2)}{x}\,dx \; = \; \left(\int_0^{\frac12} + \int_{\frac12}^1\right)\,\frac{\ln(1 + x(1-x))}{x}\,dx \\ & = & \displaystyle \int_0^{\frac12} \frac{\ln(1 + x(1-x))}{x}\,dx + \int_0^{\frac12}\frac{\ln(1 + x(1-x))}{1-x}\,dx \\ & = & \displaystyle\int_0^{\frac12} \frac{\ln(1 + x(1-x))}{x(1-x)}\,dx \\ & = & \displaystyle \int_0^{\frac12} \left(\sum_{m=0}^\infty \frac{(-1)^m}{m+1} x^m(1-x)^m\right)\,dx \; = \; \sum_{m=0}^\infty \frac{(-1)^m}{m+1}\int_0^{\frac12} x^m(1-x)^m\,dx \\ & = & \displaystyle\sum_{m=0}^\infty \frac{(-1)^m}{m+1} \int_0^{\frac14\pi} \sin^{2m}\theta \cos^{2m}\theta\,2\sin\theta\cos\theta\,d\theta \\ & = &\displaystyle 2\sum_{m=0}^\infty \frac{(-1)^m}{m+1}\int_0^{\frac14\pi} \sin^{2m+1}\theta \cos^{2m+1}\theta\,d\theta \; = \; 2\sum_{m=0}^\infty \frac{(-1)^m}{(m+1) 2^{2m+1}}\int_0^{\frac14\pi} \sin^{2m+1}2\theta\,d\theta \\ & = & \displaystyle \sum_{m=0}^\infty \frac{(-1)^m}{(m+1) 2^{2m+1}} \int_0^{\frac12\pi} \sin^{2m+1}\theta\,d\theta \; = \; \sum_{m=0}^\infty \frac{(-1)^m}{(m+1)2^{2m+1}} \frac{(2m)!!}{(2m+1)!!} \end{array} using the substitution x = sin 2 θ x = \sin^2\theta . Looking this final series up in Gradshteyn and Ryzhik (well, G&R give the formula for ( sin 1 x ) 2 (\sin^{-1}x)^2 , but close enough) gives I = 2 [ sinh 1 1 2 ] 2 = 2 [ ln ( 1 2 ( 1 + 5 ) ) ] 2 , I \; = \; 2\Big[\sinh^{-1}\tfrac12\Big]^2 \; = \; 2\Big[\ln\big(\tfrac12(1 + \sqrt{5})\big)\Big]^2 \;, so that A = 1 2 ( 1 + 5 ) A \,=\, \tfrac12(1 + \sqrt{5}) . Since A 2 = A + 1 A^2 \,=\, A + 1 , we have A 3 = A 2 + A = 2 A + 1 A^3 \,=\, A^2 + A \,=\, 2A + 1 , so that A 3 3 = 2 ( A 1 ) A^3 - 3 \,=\, 2(A-1) . Thus A ( A 3 2 ) = 2 A ( A 1 ) = 2 A(A^3-2) \,=\, 2A(A-1) \,=\, \boxed{2} .

I am sure that there must be a cute substitution that gets all the way to the last step, without the need for infinite series.

Great solution , I am sure there must be a method using dilogarithms.

Tanishq Varshney - 5 years, 4 months ago

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I tried usingvdilogatithms but I am getting 0....don't know Y???can u try and post a solution using dilogarithms

incredible mind - 5 years, 4 months ago
Dhruva Patil
Jan 31, 2016

I = 0 1 l n ( 1 + x x 2 ) x d x R o o t s o f ( 1 + x x 2 ) a r e x = 1 ± 5 2 l e t a = 1 + 5 2 b = 1 5 2 = 1 + 5 2 I = 0 1 l n [ ( x + a ) ( b x ) ] x d x I = 0 1 l n ( 1 + x a ) + l n ( 1 x b ) x d x ( l n ( a b ) = 0 s i n c e a b = 1 ) I = 0 1 l n ( 1 + x a ) x + l n ( 1 x b ) x d x U s i n g t a y l o r s e x p a n s i o n I = 0 1 1 a 1 ( 1 ) n 1 ( x a ) n 1 n 2 d x 0 1 1 b 1 ( x b ) n 1 n 2 d x I = 1 ( 1 a ) n n 2 1 ( 1 b ) n n 2 I = L i 2 ( 1 a ) L i 2 ( 1 b ) [ L i = P o l y l o g a r i t h m i c f u n c t i o n ] I 0.463130 2 ( l n ( A ) ) 2 = 0.46130 A = 0.618034 o r 1.618034 I f A = 0.618034 A n s = 1.708195 I f A = 1.618034 A n s = 2 [ C a n t f i n d a p r o p e r r e a s o n w h y s e c o n d o n e i s r i g h t a n d n o t t h e f i r s t o n e , c o m m e n t s a p p r e c i a t e d ] A n s = 2 I=\int _{ 0 }^{ 1 }{ \frac { ln(1+x-{ x }^{ 2 }) }{ x } dx } \\ Roots\quad of\quad (1+x-{ x }^{ 2 })\quad are\\ x=\frac { -1\pm \sqrt { 5 } }{ -2 } \\ let\quad \\ a=\frac { -1+\sqrt { 5 } }{ 2 } \\ b=\frac { -1-\sqrt { 5 } }{ -2 } =\frac { 1+\sqrt { 5 } }{ 2 } \\ I=\int _{ 0 }^{ 1 }{ \frac { ln[(x+a)(b-x)] }{ x } dx } \\ I=\int _{ 0 }^{ 1 }{ \frac { ln(1+\frac { x }{ a } )+ln(1-\frac { x }{ b } ) }{ x } dx } \\ (ln(ab)=0\quad since\quad ab=1)\\ I=\int _{ 0 }^{ 1 }{ \frac { ln(1+\frac { x }{ a } ) }{ x } +\quad \frac { ln(1-\frac { x }{ b } ) }{ x } dx } \\ Using\quad taylors\quad expansion\\ I=\int _{ 0 }^{ 1 }{ \frac { 1 }{ a } \sum _{ 1 }^{ \infty }{ \frac { { { (-1) }^{ n-1 }\left( \frac { x }{ a } \right) }^{ n-1 } }{ { n }^{ 2 } } } } dx-\int _{ 0 }^{ 1 }{ \frac { 1 }{ b } \sum _{ 1 }^{ \infty }{ \frac { { \left( \frac { x }{ b } \right) }^{ n-1 } }{ { n }^{ 2 } } } } dx\\ I=-\sum _{ 1 }^{ \infty }{ \frac { { \left( -\frac { 1 }{ a } \right) }^{ n } }{ { n }^{ 2 } } } -\sum _{ 1 }^{ \infty }{ \frac { { \left( \frac { 1 }{ b } \right) }^{ n } }{ { n }^{ 2 } } } \\ I=-{ Li }_{ 2 }\left( -\frac { 1 }{ a } \right) -{ Li }_{ 2 }\left( \frac { 1 }{ b } \right) \quad \quad [Li=Polylogarithmic\quad function]\\ I\simeq 0.463130\\ 2\left( ln(A) \right) ^{ 2 }=0.46130\\ A=0.618034\quad or\quad 1.618034\\ If\quad A=0.618034\\ Ans=-1.708195\\ If\quad A=1.618034\\ Ans=2\\ [Can't\quad find\quad a\quad proper\quad reason\quad why\quad second\quad one\quad is\quad right\quad \\ and\quad not\quad the\quad first\quad one,\quad \\ comments\quad appreciated]\\ Ans=\boxed { 2 }

You are correct....there are two answers. it is because (ln(1/x))^2=(ln(x))^2. I did it by complex analysis and got the answer as 2arcsin(i/2) where i is the square root of negative 1.

Arghyadeep Chatterjee - 6 months, 4 weeks ago

Thanks. I've updated the problem statement to reflect this. If we didn't specify that we're looking for the positive value of A ( A 3 3 ) A(A^3 - 3) , the other solution is approximately 1.7082. -1.7082.

In the future, if you have concerns about a problem's wording/clarity/etc., you can report the problem. See how here .

Brilliant Mathematics Staff - 6 months, 2 weeks ago

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