A definite integral...

Level 2

Find the value (up to 2 2 decimal places) of

0 1 cot 1 ( x 2 x + 1 ) d x \displaystyle\int_{0}^{1} \cot^{-1} (x^2-x+1) \ dx


The answer is 0.8776.

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1 solution

Ronak Agarwal
Aug 6, 2014

First we will write :

cot 1 ( x 2 x + 1 ) = tan 1 1 ( x 2 x + 1 ) = tan 1 1 1 + x ( x 1 ) = tan 1 x ( x 1 ) 1 + x ( x 1 ) = tan 1 x tan 1 ( x 1 ) \cot ^{ -1 }{ { (x }^{ 2 }-x+1) } =\tan ^{ -1 }{ \frac { 1 }{ ({ x }^{ 2 }-x+1) } } =\tan ^{ -1 }{ \frac { 1 }{ 1+x(x-1) } } \\ =\tan ^{ -1 }{ \frac { x-(x-1) }{ 1+x(x-1) } } =\tan ^{ -1 }{ x } -\tan ^{ -1 }{ (x-1) }

Our integral becomes I = 0 1 cot 1 ( x 2 x + 1 ) d x = 0 1 tan 1 x d x 0 1 tan 1 ( x 1 ) d x I=\int _{ 0 }^{ 1 }{ \cot ^{ -1 }{ { (x }^{ 2 }-x+1)dx } } =\int _{ 0 }^{ 1 }{ \tan ^{ -1 }{ x } dx } -\int _{ 0 }^{ 1 }{ \tan ^{ -1 }{ (x-1) } dx }

Also in the integral 0 1 tan 1 ( x 1 ) d x \int _{ 0 }^{ 1 }{ \tan ^{ -1 }{ (x-1) } dx } we put t = 1 x t=1-x we get it as : 0 1 tan 1 t d t -\int _{ 0 }^{ 1 }{ \tan ^{ -1 }{ t } dt }

I = 2 0 1 tan 1 x d x I=2\int _{ 0 }^{ 1 }{ \tan ^{ -1 }{ x } dx }

Put x = t a n θ x=tan\theta to get I = 2 0 π / 4 θ s e c 2 θ d θ I=2\int _{ 0 }^{ \pi /4 }{ \theta { sec }^{ 2 }\theta d\theta }

Apply integration by parts to get I = 2 ( θ t a n θ 0 π / 4 0 π / 4 t a n θ d θ ) = 2 ( π 4 1 2 l n ( 2 ) ) = π 2 l n ( 2 ) I=2(\theta tan\theta { | }_{ 0 }^{ \pi /4 }-\int _{ 0 }^{ \pi /4 }{ tan\theta d\theta } )=2(\frac { \pi }{ 4 } -\frac { 1 }{ 2 } ln(2))=\boxed { \frac { \pi }{ 2 } -ln(2) }

This is a N.C.E.R.T problem .

Just got an idea to make this problem more interesting

0 1 t a n 1 ( x 2 x + 1 ) d x \displaystyle \int_{0}^1 tan^{-1}( x^{2} - x + 1)dx

U Z - 6 years, 7 months ago

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