Seems too easy!

Calculus Level 5

0 π / 4 x 2 ( sin 2 x cos 2 x ) ( 1 + sin 2 x ) cos 2 x d x \int_{0}^{^\pi/_4} \frac{x^{2} (\sin 2x - \cos 2x) }{(1+\sin 2x)\cos^2 x} dx can be expressed as π α β π γ ln λ \frac{\pi^{\alpha}}{\beta} - \frac{\pi}{\gamma} \ln \lambda .What is the value of α + β + γ + λ \alpha + \beta + \gamma + \lambda ?

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The answer is 24.

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1 solution

Abhishek Singh
Oct 10, 2014

OK Here we begin … Let 2 x = t \displaystyle 2x = t ,then d x = 1 2 d t \displaystyle dx = \frac{1}{2} dt ,and our integral changes as I = 1 8 0 π / 2 t 2 ( sin t cos t ) ( 1 + sin t ) ( cos 2 ( t / 2 ) ) d x I = \frac{1}{8} \int_{0}^{^\pi/_2} \dfrac{t^2 (\sin t - \cos t)}{(1 + \sin t)(\cos^2 (^t/_2))} dx I = 1 4 0 π / 2 t 2 ( sin t cos t ) ( 1 + sin t ) ( 1 + cos t ) d x . ( 1 ) I = \dfrac{1}{4} \int_{0}^{^\pi/_2} \dfrac{t^2 (\sin t - \cos t)}{(1 + \sin t)(1 + \cos t)} dx………….(1) Now using the properties of definite integral we get I = 1 4 0 π / 2 ( π 2 t ) 2 ( cos t sin t ) ( 1 + cos t ) ( 1 + sin t ) d x . . . . . ( 2 ) I = \dfrac{1}{4} \int_{0}^{^\pi/_2} \dfrac{\Bigg( \dfrac{\pi}{2} - t\Bigg)^{2} (\cos t - \sin t)}{(1 + \cos t)(1 + \sin t)} dx…………….....(2) Adding (1)&(2) we get 2 I = 1 4 0 π / 2 ( π t π 2 4 ) ( sin t cos t ) ( 1 + sin t ) ( 1 + cos t ) d x 2I = \dfrac{1}{4} \int_{0}^{^\pi/_2} \dfrac{\Bigg( \pi t - \dfrac{\pi^{2}}{4}\Bigg) (\sin t - \cos t)}{(1 + \sin t)(1 + \cos t)} dx But 0 π / 2 sin t cos t ( 1 + sin t ) ( 1 + cos t ) d x = 0 \int_{0}^{^\pi/_{2}} \dfrac{\sin t - \cos t}{(1 + \sin t)(1 + \cos t) } dx = 0 And hence after distributing we get I = 1 8 0 π / 2 π t ( sin t cos t ) ( 1 + sin t ) ( 1 + cos t ) d x I = \dfrac{1}{8} \int_{0}^{^\pi/_2} \dfrac{\pi t (\sin t - \cos t)}{(1 + \sin t)(1 + \cos t) } dx Hence after solving this we get I = π 2 16 π 4 log 2 I = \boxed{ \dfrac{\pi^2}{16} - \dfrac{\pi}{4} \log2 }

Well I did by using Properties and By parts

Deepanshu Gupta - 6 years, 7 months ago

did same !!

Rishabh Jain - 6 years, 8 months ago

I can't for the world of me imagine how you are integrating an expression in t w.r.t. x !!! (My solution was almost same, but without substitution, so in my case, dx was right. ; )

Aakarshit Uppal - 6 years, 4 months ago

a few typos in the solution. Replace dx by dt.

Ankit Kumar Jain - 3 years ago

May someone explain the last integral more, I could not get it... Thanks

Ivan Wang - 5 years, 7 months ago

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