Now this is easier right?

Calculus Level 5

0 π 2 x ln 2 ( sin 2 x ) d x = π A B ln C ( D ) + π E F \large \displaystyle \int_0^{\frac{\pi}{2}} x\ln^{2}(\sin 2x)\mathbb{d}x = \dfrac{\pi^{A}}{B}\ln^{C}(D) +\dfrac{\pi^{E}}{F} If the above integral is true for positive integers: A , B , C , D , E , F A,B,C,D,E,F
and D D is a prime, find:
A + B + C + D + E + F A+B+C+D+E+F


Try its harder version.


The answer is 114.

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1 solution

Kunal Gupta
Oct 3, 2015

The answer is: π 2 8 ln 2 ( 2 ) + π 4 96 \dfrac{\pi^{2}}{8}\ln^{2}(2) +\dfrac{\pi^{4}}{96} The SOLUTION \text{SOLUTION}
I = 0 π 2 x ln 2 ( sin 2 x ) d x I =\displaystyle \int_0^{\frac{\pi}{2}} x\ln^{2}(\sin 2x)\mathbb{d}x Using f ( a + b x ) = f ( x ) f(a+b-x) =f(x) and adding the original and transformed integral, we get: I = π 4 0 π 2 ln 2 ( s i n ( 2 x ) ) d x = π 4 0 π 2 ln 2 ( 2 sin ( x ) cos ( x ) ) d x I =\dfrac{\pi}{4} \displaystyle \int_{0}^{\frac{\pi}{2}}\ln^2(sin (2x)) dx \\ =\dfrac{\pi}{4} \displaystyle \int_{0}^{\frac{\pi}{2}}\ln^2(2\sin(x)\cos(x)) dx \\ Expanding the square term and separating terms; we get I = 0 π 2 [ ln 2 ( 2 ) + 2 ln 2 ( sin ( x ) ) + 4 ln ( 2 ) ln ( sin ( x ) ) + 2 ln ( sin ( x ) ) ln ( cos ( x ) ) ] d x I =\displaystyle \int_{0}^{\frac{\pi}{2}}\left[ \ln^{2}(2) + 2\ln^{2}(\sin(x))+ 4\ln(2)\ln(\sin(x)) +2\ln(\sin(x))\ln(\cos(x)) \right]dx where the fact that sin ( x ) = cos ( π 2 x ) \sin(x) =\cos(\dfrac{\pi}{2}- x) is used.
Further evaluating elementary integrals, and that integral of logarithm of sine and cosine over 0 0 to π / 2 \pi/2 is π 2 ln ( 2 ) -\dfrac{\pi}{2}\ln(2) , we get: I = 3 8 π 2 ln 2 ( 2 ) + π 2 [ 0 π / 2 ln 2 ( sin ( x ) ) d x K + 0 π / 2 ln ( sin ( x ) ) ln ( cos ( x ) ) d x L ] I = -\dfrac{3}{8}\pi^{2}\ln^{2}(2) + \dfrac{\pi}{2}\left[ \underbrace{\displaystyle \int_{0}^{\pi/2}\ln^{2}(\sin(x))dx}_{K} +\underbrace{\displaystyle \int_{0}^{\pi/2}\ln(\sin(x))\ln(\cos(x))dx}_{L} \right] Now to evaluate these integrals, the main work starts: Consider the following integral ( a , b ) = 0 π / 2 sin a ( x ) cos b ( x ) d x = Γ ( a + 1 2 ) Γ ( b + 1 2 ) 2 Γ ( a + b + 2 2 ) \Im(a,b) = \displaystyle \int_{0}^{\pi/2} \sin^{a}(x)\cos^{b}(x) dx = \dfrac{\Gamma(\frac{a+1}{2})\Gamma(\frac{b+1}{2})}{2\Gamma(\frac{a+b+2}{2})} Differentiating above partially first w.rt a a and then w.r.t b b , we get ( a , b ) a b = 0 π / 2 sin a ( x ) cos b ( x ) ln ( sin ( x ) ) ln ( cos ( x ) ) d x = 1 8 Γ ( 1 + a 2 ) Γ ( b + 1 2 ) Γ ( a + b + 2 2 ) [ ψ ( a + b + 2 2 ) + [ ψ ( b + 1 2 ) ψ ( a + b + 2 2 ) ] [ ψ ( 1 + a 2 ) ψ ( a + b + 2 2 ) ] ] \dfrac{\partial \Im(a,b)}{\partial a \partial b} = \displaystyle \int_{0}^{\pi/2} \sin^{a}(x)\cos^{b}(x)\ln(\sin(x))\ln(\cos(x)) dx\\ = \dfrac{1}{8}\dfrac{\Gamma(\frac{1+a}{2})\Gamma(\frac{b+1}{2})}{\Gamma(\frac{a+b+2}{2})}\left[ -\psi'(\dfrac{a+b+2}{2}) + \left[ \psi(\frac{b+1}{2}) -\psi(\frac{a+b+2}{2})\right]\left[\psi(\frac{1+a}{2}) -\psi(\frac{a+b+2}{2})\right] \right] where Γ ( x ) \Gamma(x) is the Gamma Function and ψ ( x ) \psi(x) and ψ ( x ) \psi'(x) are the Digamma and Trigamma Functions Respectively.
Now, Plugging a = 0 a=0 and b = 0 b=0 , we get: L = 0 π / 2 ln ( sin ( x ) ) ln ( cos ( x ) ) d x = π 3 48 + π 2 ln 2 ( 2 ) L =\displaystyle \int_{0}^{\pi/2} \ln(\sin(x))\ln(\cos(x)) dx = -\dfrac{\pi^3}{48} +\dfrac{\pi}{2}\ln^{2}(2) where, the fact that: Γ ( 1 / 2 ) = π \Gamma(1/2) =\sqrt{\pi} , ψ ( 1 ) = γ \psi(1) = -\gamma , ψ ( 1 / 2 ) = 2 ln ( 2 ) γ \psi(1/2) =-2\ln(2) -\gamma and ψ ( 1 ) = π 2 6 \psi'(1) =\dfrac{\pi^{2}}{6} are used. Here, γ \gamma is the Euler Mascheroni Constant.
Now H n H_{n} is the Harmonic Number and
For evaluating ψ ( 1 ) \psi'(1) , the following consideration was used: ψ ( n ) = H n 1 γ \psi(n) =H_{n-1} -\gamma Differentiating w.r.t n n ; ψ ( n ) = ( H n 1 ) \psi'(n) = (H_{n-1})' but as H n 1 = 0 1 1 x n 2 1 x d x H_{n-1} =\displaystyle \int_{0}^{1} \dfrac{1-x^{n-2}}{1-x}dx ψ ( n \psi'(n can be calculated. Now , for calculating K K , the integral: ( a ) = 0 π / 2 sin a ( x ) d x \Im(a) = \displaystyle \int_{0}^{\pi/2} \sin^{a}(x) dx is used and is Differentiated twice.This has been left as an exercise to the reader. The intregral comes out to be: K = 0 π / 2 ln 2 ( sin ( x ) ) d x = π 2 ln 2 ( 2 ) + π 3 24 K =\displaystyle \int_{0}^{\pi/2}\ln^{2}(\sin(x))dx =\dfrac{\pi}{2}\ln^{2}(2) + \dfrac{\pi^{3}}{24} So, the final Integral Comes out to be: 0 π 2 x ln 2 ( sin 2 x ) d x = π 2 8 ln 2 ( 2 ) + π 4 96 \large \boxed{\displaystyle \int_0^{\frac{\pi}{2}} x\ln^{2}(\sin 2x)\mathbb{d}x = \dfrac{\pi^{2}}{8}\ln^{2}(2) +\dfrac{\pi^{4}}{96 } }


Erm. How? Can you show your working so I print it out ;)

Pi Han Goh - 5 years, 8 months ago

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@Pi Han Goh now you happy!!?? ;)

Kunal Gupta - 5 years, 8 months ago

@Pi Han Goh Did you print !!?? :P and is the answer clean??

Kunal Gupta - 5 years, 8 months ago

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I'm still reading it. It's not easy to read through it when I don't know integrals.

Pi Han Goh - 5 years, 8 months ago

Nice problem @Upanshu Gupta . I learnt about digamma function. I searched the internet for the proof of the following relation ψ ( x ) = 0 ( e t t e x t 1 e t ) d t \psi (x) = \int _{0}^{\infty}\left(\dfrac{e^{-t}}{t} - \dfrac{e^{-xt}}{1-e^{-t}} \right) dt and ψ ( n ) = H n 1 γ \psi (n) = H_{n-1} - \gamma . But I didnt find the proof anywhere. Can you please let me know the proof? Or let me know the better where I can find proof for all these types of Transcendental Functions.

Surya Prakash - 5 years, 8 months ago

Yep. It's correct. I've read it over 15 time and I can't find a mistake. Let me on my printer! Thanks!

Pi Han Goh - 5 years, 8 months ago

now solve this . don't give the answer just solve with a solution

0 π / 2 x log 2 ( s i n x ) d x \displaystyle \int\limits_{0}^{\pi/2} x\log^2(sinx) dx

Aman Rajput - 5 years, 7 months ago

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