∫ 0 1 x ln ( 1 + x − x 2 ) d x
If the value of the integral above is equal to 2 ( ln A ) 2 , find the positive value of A ( A 3 − 3 ) .
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
Great solution , I am sure there must be a method using dilogarithms.
Log in to reply
I tried usingvdilogatithms but I am getting 0....don't know Y???can u try and post a solution using dilogarithms
I = ∫ 0 1 x l n ( 1 + x − x 2 ) d x R o o t s o f ( 1 + x − x 2 ) a r e x = − 2 − 1 ± 5 l e t a = 2 − 1 + 5 b = − 2 − 1 − 5 = 2 1 + 5 I = ∫ 0 1 x l n [ ( x + a ) ( b − x ) ] d x I = ∫ 0 1 x l n ( 1 + a x ) + l n ( 1 − b x ) d x ( l n ( a b ) = 0 s i n c e a b = 1 ) I = ∫ 0 1 x l n ( 1 + a x ) + x l n ( 1 − 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 a 1 ∑ 1 ∞ n 2 ( − 1 ) n − 1 ( a x ) n − 1 d x − ∫ 0 1 b 1 ∑ 1 ∞ n 2 ( b x ) n − 1 d x I = − ∑ 1 ∞ n 2 ( − a 1 ) n − ∑ 1 ∞ n 2 ( b 1 ) n I = − L i 2 ( − a 1 ) − L i 2 ( b 1 ) [ 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 . 4 6 3 1 3 0 2 ( l n ( A ) ) 2 = 0 . 4 6 1 3 0 A = 0 . 6 1 8 0 3 4 o r 1 . 6 1 8 0 3 4 I f A = 0 . 6 1 8 0 3 4 A n s = − 1 . 7 0 8 1 9 5 I f A = 1 . 6 1 8 0 3 4 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
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.
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 ) , the other solution is approximately − 1 . 7 0 8 2 .
In the future, if you have concerns about a problem's wording/clarity/etc., you can report the problem. See how here .
Problem Loading...
Note Loading...
Set Loading...
We start with a bit of variable manipulation: I = = = = = = = ∫ 0 1 x ln ( 1 + x − x 2 ) d x = ( ∫ 0 2 1 + ∫ 2 1 1 ) x ln ( 1 + x ( 1 − x ) ) d x ∫ 0 2 1 x ln ( 1 + x ( 1 − x ) ) d x + ∫ 0 2 1 1 − x ln ( 1 + x ( 1 − x ) ) d x ∫ 0 2 1 x ( 1 − x ) ln ( 1 + x ( 1 − x ) ) d x ∫ 0 2 1 ( m = 0 ∑ ∞ m + 1 ( − 1 ) m x m ( 1 − x ) m ) d x = m = 0 ∑ ∞ m + 1 ( − 1 ) m ∫ 0 2 1 x m ( 1 − x ) m d x m = 0 ∑ ∞ m + 1 ( − 1 ) m ∫ 0 4 1 π sin 2 m θ cos 2 m θ 2 sin θ cos θ d θ 2 m = 0 ∑ ∞ m + 1 ( − 1 ) m ∫ 0 4 1 π sin 2 m + 1 θ cos 2 m + 1 θ d θ = 2 m = 0 ∑ ∞ ( m + 1 ) 2 2 m + 1 ( − 1 ) m ∫ 0 4 1 π sin 2 m + 1 2 θ d θ m = 0 ∑ ∞ ( m + 1 ) 2 2 m + 1 ( − 1 ) m ∫ 0 2 1 π sin 2 m + 1 θ d θ = m = 0 ∑ ∞ ( m + 1 ) 2 2 m + 1 ( − 1 ) m ( 2 m + 1 ) ! ! ( 2 m ) ! ! using the substitution x = sin 2 θ . Looking this final series up in Gradshteyn and Ryzhik (well, G&R give the formula for ( sin − 1 x ) 2 , but close enough) gives I = 2 [ sinh − 1 2 1 ] 2 = 2 [ ln ( 2 1 ( 1 + 5 ) ) ] 2 , so that A = 2 1 ( 1 + 5 ) . Since A 2 = A + 1 , we have A 3 = A 2 + A = 2 A + 1 , so that A 3 − 3 = 2 ( A − 1 ) . Thus A ( A 3 − 2 ) = 2 A ( A − 1 ) = 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.