It's not like the others

Calculus Level 5

0 x 21 + x 20 + x 19 + + x 2 + x + 1 x 23 + x 22 + x 21 + + x 2 + x + 1 d x \large \int_0^\infty \frac{x^{21}+x^{20} +x^{19}+ \ldots+x^2 +x+1}{x^{23} + x^{22}+x^{21} +\ldots+x^2+x+1} \, dx

Given that the integral above equals to π A ( 1 B + 1 C + 1 D + 1 E ) \dfrac{\pi}{\sqrt A} \left( \dfrac1{\sqrt B}+\dfrac1{\sqrt C}+\dfrac1{\sqrt D}+\dfrac1{\sqrt E} \right) for positive integers A , B , C , D A,B,C,D and E E .

Find the minimum value of B × C × D × E ÷ A B\times C\times D \times E \div A .

Inspiration .


The answer is 12.

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2 solutions

Kartik Sharma
Aug 13, 2015

0 1 x 22 1 x 24 d x \displaystyle \int_{0}^{\infty}{\frac{1-{x}^{22}}{1-{x}^{24}} dx}

0 1 1 x 24 d x 0 x 22 1 x 24 d x \displaystyle \int_{0}^{\infty}{\frac{1}{1-{x}^{24}} dx} - \int_{0}^{\infty}{\frac{{x}^{22}}{1-{x}^{24}} dx}

I 1 = 0 1 1 x 24 d x , I 2 = 0 x 22 1 x 24 d x \displaystyle {I}_{1} = \int_{0}^{\infty}{\frac{1}{1-{x}^{24}} dx}, {I}_{2} = \int_{0}^{\infty}{\frac{{x}^{22}}{1-{x}^{24}} dx}

I 1 : x 24 x \displaystyle {I}_{1} : \quad {x}^{24}\rightarrow x

1 24 0 x 23 / 24 1 x d x \displaystyle \frac{1}{24}\int_{0}^{\infty}{\frac{{x}^{-23/24}}{1-x} dx}

1 24 M ( 1 1 x ) ( 1 24 ) \displaystyle \frac{1}{24} M\left(\frac{1}{1-x}\right)\left(\frac{1}{24}\right)

M ( f ) ( s ) M(f)(s) - Mellin transform of f f at s s

Using Ramanujan Master Theorem,

1 24 Γ ( 1 24 ) Γ ( 1 1 24 ) ( 1 ) 1 / 24 \displaystyle \frac{1}{24} \Gamma\left(\frac{1}{24}\right)\Gamma\left(1 - \frac{1}{24}\right){(-1)}^{1/24}

Using Euler's Reflection Formula,

1 24 π s i n ( π 24 ) e π ι / 24 \displaystyle \frac{1}{24} \frac{\pi}{sin\left(\frac{\pi}{24}\right)} {e}^{\pi\iota/24}

I 2 : x 24 x \displaystyle {I}_{2} : \quad {x}^{24}\rightarrow x

1 24 0 x 1 / 24 1 x d x \displaystyle \frac{1}{24}\int_{0}^{\infty}{\frac{{x}^{-1/24}}{1-x} dx}

1 24 M ( 1 1 x ) ( 23 24 ) \displaystyle \frac{1}{24} M\left(\frac{1}{1-x}\right)\left(\frac{23}{24}\right)

Proceeding as before,

1 24 π s i n ( 23 π 24 ) e 23 π ι / 24 \displaystyle \frac{1}{24} \frac{\pi}{sin\left(\frac{23\pi}{24}\right)} {e}^{23\pi\iota/24}

Putting them in one,

π 24 s i n ( π 24 ) ( e π ι / 24 + e π ι / 24 ) \displaystyle \frac{\pi}{24 sin\left(\frac{\pi}{24}\right)}\left({e}^{\pi\iota/24} + {e}^{-\pi\iota/24}\right)

π 12 c o t ( π 24 ) \displaystyle \frac{\pi}{12} cot\left(\frac{\pi}{24}\right)

π 12 ( 2 + 6 + 3 + 2 ) \displaystyle \frac{\pi}{12} \left(2 + \sqrt{6} + \sqrt{3} + \sqrt{2}\right)

π 12 ( 1 3 + 1 2 + 1 4 + 1 6 ) \displaystyle \frac{\pi}{\sqrt{12}}\left(\frac{1}{\sqrt{3}} + \frac{1}{\sqrt{2}} + \frac{1}{\sqrt{4}} + \frac{1}{\sqrt{6}}\right)

@Pi Han Goh I hope that's a fine solution(non-English). Another use of the Master! I'm lovin it.

Kartik Sharma - 5 years, 10 months ago

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Pi Han Goh - 5 years, 10 months ago

Even I used the same method. R u really 15, cuz ur solution make me think u r a phd at maths

Aditya Kumar - 5 years, 10 months ago

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Of course I am 15. But if my solution make you think I am a PhD at Maths, then I'd think the same for you as you also used the same method and are less than an year older than me.

Kartik Sharma - 5 years, 10 months ago

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@Kartik Sharma Actually I'm going to be 17 next moth. I've seen many of ur solutions. They r really amazing!

Aditya Kumar - 5 years, 10 months ago

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@Aditya Kumar Oh, thanks. BTW, I'm also going to be 16 next month.

You're really good at Mechanics and EM as I have seen. Just amazing.

Kartik Sharma - 5 years, 10 months ago

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@Kartik Sharma Haha thanks! Btw maths is the mother of all.

Aditya Kumar - 5 years, 10 months ago

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@Aditya Kumar This is why. Mine is father's.

Lu Chee Ket - 5 years, 7 months ago

One can find the general integral of such form in the same way. I will give the generalized form also.

Kartik Sharma - 5 years, 10 months ago

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POST POST POST!!!

Pi Han Goh - 5 years, 10 months ago

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Just the generalized form -

0 1 x a 1 x b d x = π a ( e π ι / a s i n ( π a ) e b π ι / a s i n ( ( b + 1 ) π a ) ) \displaystyle \int_{0}^{\infty}{\frac{1-{x}^{a}}{1-{x}^{b}} dx} = \frac{\pi}{a}\left(\frac{{e}^{\pi\iota/a}}{sin\left(\frac{\pi}{a}\right)} - \frac{{e}^{b\pi\iota/a}}{sin\left((b+1)\frac{\pi}{a}\right)}\right)

Proceeding with the constants a , b a,b like I did in the problem generalizes it.

Kartik Sharma - 5 years, 10 months ago

where did you learn Euler's reflection formula and the other 2 fancy theorems?

Curtis Clement - 5 years, 10 months ago

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You don't actually need to apply all these fancy theorems, see my solution given in "inspiration".

Euler's reflection formula pops up very often for gamma functions so it's useful to know it.

Pi Han Goh - 5 years, 10 months ago

how to get cot π 24 = 2 + 2 + 3 + 6 \displaystyle \cot \dfrac{\pi}{24}=2+\sqrt{2}+\sqrt{3}+\sqrt{6} ?

uzumaki nagato tenshou uzumaki - 4 years, 4 months ago
Lu Chee Ket
Oct 21, 2015

I suspected this as a wrong direction however I know that I can solve via numerical method.

I have just updated upon request.

Lu Chee Ket - 5 years, 7 months ago

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