It's Rainin' Integrals

Calculus Level 5

0 1 d x ln x = π A 0 1 ln x d x = π B 0 1 ln 2 x 1 + x 2 d x = π C D 0 ln x 1 + x 2 d x = E 0 ln x 1 + x 3 d x = B F π B 1 1 cos x e 1 x + 1 d x = sin A 0 sin 3 x x 2 d x = C G ln C 0 ( sin x x ) 5 d x = H I π 0 ( sin x x ) 6 d x = J K π \begin{aligned} \int_{0}^{1} \frac{dx}{\sqrt{-\ln{x}}} & =\frac{\sqrt{\pi}}{A} \\ \int_{0}^{1} \sqrt{-\ln{x}} dx & = \frac{\sqrt{\pi}}{B} \\ \int_{0}^{1} \frac{\ln^{2}x}{1+x^2} dx & =\frac{\pi^{C}}{D} \\ \int_{0}^{\infty} \frac{\ln x}{1+x^2} dx & =E \\ \int_{0}^{\infty} \frac{\ln x}{1+x^3} dx & =-\frac{B}{F}\pi^{B} \\ \int_{-1}^{1} \frac{\cos x}{e^{\frac{1}{x}}+1} dx & =\sin{A} \\ \int_{0}^{\infty} \frac{\sin^{3} x}{x^2} dx & =\frac{C}{G}\ln{C} \\ \int_{0}^{\infty} \bigg(\frac{\sin x}{x}\bigg)^5 dx & =\frac{H}{I}\pi \\ \int_{0}^{\infty} \bigg(\frac{\sin x}{x}\bigg)^6 dx & =\frac{J}{K}\pi \end{aligned}

Here, A , B , C , D , E , F , G , H , I , J , K A,B,C,D,E,F,G,H,I,J,K are distinct positive integers and all the fractions are in their simplest forms.

Find A + B + C + D + E + F + G + H + I + J + K A+B+C+D+E+F+G+H+I+J+K


The answer is 603.

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1 solution

Chew-Seong Cheong
Oct 18, 2017

0 1 d x ln x = 0 1 d x ln 1 x Let u = 1 x d u = d x x 2 = 1 d u u 2 ln u Let t = ln u e t d t = d u = 0 t 1 2 1 e t d t = Γ ( 1 2 ) = π where Γ ( ) denotes the gamma function. \begin{aligned} \int_0^1 \frac {dx}{\sqrt{-\ln x}} & = \int_0^1 \frac {dx}{\sqrt{\ln \frac 1x}} & \small \color{#3D99F6} \text{Let }u = \frac 1x \implies du = - \frac {dx}{x^2} \\ & = - \int_{\infty}^1 \frac {du}{u^2\sqrt{\ln u}} & \small \color{#3D99F6} \text{Let }t = \ln u \implies e^t \ dt = du \\ & = \int_0^{\infty} t^{\frac 12 - 1}e^{-t} dt \\ & = \Gamma \left(\frac 12\right) = \sqrt \pi & \small \color{#3D99F6} \text{where }\Gamma (\cdot) \text{ denotes the gamma function.} \end{aligned}

A = 1 \implies A = 1


0 1 ln x d x = 0 1 ln 1 x d x Let u = 1 x d u = d x x 2 = 1 ln u u 2 d u Let t = ln u e t d t = d u = 0 t 3 2 1 e t d t = Γ ( 3 2 ) = 1 2 Γ ( 1 2 ) = π 2 \begin{aligned} \int_0^1 \sqrt{-\ln x} \ dx & = \int_0^1 \sqrt{\ln \frac 1x} \ dx & \small \color{#3D99F6} \text{Let }u = \frac 1x \implies du = - \frac {dx}{x^2} \\ & = - \int_{\infty}^1 \frac {\sqrt{\ln u}}{u^2}du & \small \color{#3D99F6} \text{Let }t = \ln u \implies e^t \ dt = du \\ & = \int_0^{\infty} t^{\frac 32 - 1}e^{-t} dt \\ & = \Gamma \left(\frac 32\right) = \frac 12 \Gamma \left(\frac 12\right) = \frac {\sqrt \pi}2 \end{aligned}

B = 2 \implies B = 2


0 1 ln 2 x 1 + x 2 d x = 0 1 ln 2 x ( x i ) ( x + i ) d x By partial fraction = 0 1 ( i ln 2 x 2 ( x + i ) i ln 2 x 2 ( x i ) ) d x Let u = x + i , v = x i = i 2 ( i 1 + i ln 2 ( u i ) u d u i 1 i ln 2 ( v + i ) v d v ) By integration by parts = i 2 ( ln ( i u ) ln 2 ( u i ) i 1 + i 0 2 i 1 + i ln ( i u ) ln ( u i ) u i d u ln ( i v ) ln 2 ( v + i ) i 1 i 0 + 2 i 1 i ln ( i v ) ln ( v + i ) v + i d v ) = i ( i 1 + i ln ( i u ) ln ( u i ) u i d u + i 1 i ln ( i v ) ln ( v + i ) v + i d v ) By integration by parts = i ( ln ( u i ) Li 2 ( i u + 1 ) i 1 + i 0 i 1 + i Li 2 ( i u + 1 ) ln ( u i ) d u ln ( v + i ) Li 2 ( 1 i v ) i 1 i 0 + i 1 i Li 2 ( 1 i v ) ln ( v + i ) d v ) Li s ( z ) is polylogarithm function. = i ( i 1 + i Li 2 ( i u + 1 ) ln ( u i ) d u + i 1 i Li 2 ( 1 i v ) ln ( v + i ) d v ) = i ( Li 3 ( i u + 1 ) i 1 + i + Li 3 ( 1 i v ) i 1 i ) = i ( Li 3 ( i ) + Li 3 ( 0 ) + Li 3 ( i ) Li 3 ( 0 ) ) = π 3 16 \begin{aligned} \int_0^1 \frac {\ln^2 x}{1+x^2} \ dx & = \int_0^1 \frac {\ln^2 x}{(x-i)(x+i)} \ dx & \small \color{#3D99F6} \text{By partial fraction} \\ & = \int_0^1 \left(\frac {i\ln^2 x}{2(x+i)} - \frac {i\ln^2 x}{2(x-i)}\right) dx & \small \color{#3D99F6} \text{Let }u = x+i, \ v = x-i \\ & = \frac i2 \left(\int_i^{1+i} \frac {\ln^2 (u-i)}u du - \int_{-i}^{1-i} \frac {\ln^2 (v+i)}v dv \right) & \small \color{#3D99F6} \text{By integration by parts} \\ & = \frac i2 \bigg(\cancel{\ln(-iu)\ln^2(u-i)\bigg|_i^{1+i}}^0 - 2\int_i^{1+i} \frac {\ln(-iu)\ln (u-i)}{u-i} du \\ & \quad \quad - \cancel{\ln(iv)\ln^2(v+i)\bigg|_{-i}^{1-i}}^0 + 2\int_{-i}^{1-i} \frac {\ln (iv)\ln (v+i)}{v+i} dv \bigg) \\ & = i\left(- \int_i^{1+i} \frac {\ln(-iu)\ln (u-i)}{u-i} du + \int_{-i}^{1-i} \frac {\ln (iv)\ln (v+i)}{v+i} dv\right) & \small \color{#3D99F6} \text{By integration by parts} \\ & = i \bigg(\cancel{\ln(u-i) \text{Li}_2 (iu+1) \bigg|_i^{1+i}}^0 - \int_i^{1+i} \frac {\text{Li}_2 (iu+1)}{\ln(u-i)} du \\ & \quad \quad - \cancel{\ln(v+i) \text{Li}_2 (1-iv) \bigg|_{-i}^{1-i}}^0 + \int_{-i}^{1-i} \frac {\text{Li}_2 (1-iv)}{\ln(v+i)} dv \bigg) & \small \color{#3D99F6} \text{Li}_s (z) \text{ is polylogarithm function.} \\ & = i \left(- \int_i^{1+i} \frac {\text{Li}_2 (iu+1)}{\ln(u-i)} du + \int_{-i}^{1-i} \frac {\text{Li}_2 (1-iv)}{\ln(v+i)} dv \right) \\ & = i \left(-\text{Li}_3 (iu+1) \bigg|_i^{1+i} + \text{Li}_3 (1-iv) \bigg|_{-i}^{1-i} \right) \\ & = i \left(-\text{Li}_3 (i) + \text{Li}_3 (0) + \text{Li}_3 (-i) - \text{Li}_3 (0) \right) \\ & = \frac {\pi^3}{16} \end{aligned}

C = 3 \implies C = 3 and D = 16 D = 16


0 ln x 1 + x 2 d x = 1 2 0 ( ln x 1 + x 2 + ln 1 x x 2 ( 1 + 1 x 2 ) ) d x Using 0 f ( x ) d x = 0 f ( 1 x ) x 2 d x = 1 2 0 ( ln x 1 + x 2 ln x x 2 + 1 ) d x = 0 \begin{aligned} \int_0^\infty \frac {\ln x}{1+x^2} \ dx & = \frac 12 \int_0^\infty \left(\frac {\ln x}{1+x^2} + \frac {\ln \frac 1x}{x^2\left(1+ \frac 1{x^2} \right)} \right) dx & \small \color{#3D99F6} \text{Using }\int_0^\infty f(x) \ dx = \int_0^\infty \frac {f\left(\frac 1x\right)}{x^2} dx \\ & = \frac 12 \int_0^\infty \left(\frac {\ln x}{1+x^2} - \frac {\ln x}{x^2+ 1} \right) dx \\ &= 0 \end{aligned}

E = 0 \implies E = 0


Solution too tedius to show \color{#D61F06} \text{Solution too tedius to show}

0 ln x 1 + x 3 d x = 0 ln x ( x + 1 ) ( x 2 x + 1 ) d x = 2 27 π 2 \begin{aligned} \int_0^\infty \frac {\ln x}{1+x^3} dx & = \int_0^\infty \frac {\ln x}{(x+1)(x^2-x+1)} dx = - \frac 2{27}\pi^2 \end{aligned}

F = 27 \implies F = 27


1 1 cos x e 1 x + 1 d x = 1 2 1 1 ( cos x e 1 x + 1 + cos ( x ) e 1 x + 1 ) d x Using a b f ( x ) d x = 1 2 a b f ( a + b x ) d x = 1 1 ( cos x e 1 x + 1 + e 1 x cos x 1 + e 1 x ) d x = 1 2 1 1 cos x d x = 1 2 sin x 1 1 = sin 1 \begin{aligned} \int_{-1}^1 \frac {\cos x}{e^\frac 1x +1} dx & = \frac 12 \int_{-1}^1 \left(\frac {\cos x}{e^\frac 1x +1} + \frac {\cos (-x)}{e^{-\frac 1x} +1} \right) dx & \small \color{#3D99F6} \text{Using }\int_a^b f(x) \ dx = \frac 12 \int_a^b f(a+b-x) \ dx \\ & = \int_{-1}^1 \left(\frac {\cos x}{e^\frac 1x +1} + \frac {e^\frac 1x \cos x}{1+ e^\frac 1x} \right) dx \\ & = \frac 12 \int_{-1}^1 \cos x \ dx = \frac 12 \sin x\bigg|_{-1}^1 \\ & = \sin 1 \end{aligned}


0 sin 3 x x 2 d x = 3 4 0 sin x x 2 d x 1 4 0 sin 3 x x 2 d x Using sin 3 θ = 3 sin θ 4 sin 3 θ = 3 sin x 4 x 0 + 3 4 0 cos x x d x + sin 3 x 4 x 0 1 4 0 3 cos 3 x x d x By integration by parts = 3 4 + lim x 0 3 4 Ci ( x ) + 3 4 lim x 0 3 4 Ci ( 3 x ) = 3 4 lim x 0 ( Ci ( x ) Ci ( 3 x ) ) = 3 4 ln 3 \begin{aligned} \int_0^\infty \frac {\sin^3 x}{x^2} dx & = \frac 34 \int_0^\infty \frac {\sin x}{x^2}dx - \frac 14 \int_0^\infty \frac {\sin 3x}{x^2} dx & \small \color{#3D99F6} \text{Using }\sin 3\theta = 3\sin \theta - 4 \sin^3 \theta \\ & = - \frac {3\sin x}{4x} \ \bigg|_0^\infty + \frac 34 \int_0^\infty \frac {\cos x}x dx + \frac {\sin 3x}{4x} \ \bigg|_0^\infty - \frac 14 \int_0^\infty \frac {3 \cos 3x}x dx & \small \color{#3D99F6} \text{By integration by parts} \\ & = - \frac 34 + \lim_{x \to 0} \frac 34 \text{Ci}(x) + \frac 34 - \lim_{x \to 0} \frac 34 \text{Ci}(3x) \\ & = \frac 34 \lim_{x \to 0} (\text{Ci}(x) - \text{Ci}(3x)) \\ & = \frac 34 \ln 3 \end{aligned}

G = 4 \implies G = 4


Solution too tedius to show \color{#D61F06} \text{Solution too tedius to show}

0 sin 5 x x 5 d x = 0 sin 5 x 5 sin 3 x + 10 sin x 16 x 5 d x = 115 384 π \begin{aligned} \int_0^\infty \frac {\sin^5 x}{x^5} dx & = \int_0^\infty \frac {\sin 5x-5\sin 3x+10\sin x}{16x^5} dx = \frac {115}{384}\pi\end{aligned}

H = 115 \implies H = 115 and I = 384 I = 384


Solution too tedius to show \color{#D61F06} \text{Solution too tedius to show}

0 sin 6 x x 6 d x = 0 10 15 cos 2 x + 6 cos 4 x cos 6 x 32 x 5 d x = 11 40 π \begin{aligned} \int_0^\infty \frac {\sin^6 x}{x^6} dx & = \int_0^\infty \frac {10-15\cos 2x+6 \cos 4x-\cos 6x}{32x^5} dx = \frac {11}{40}\pi\end{aligned}

J = 11 \implies J = 11 and K = 40 K = 40


A + B + C + D + E + F + G + H + I + J + K = 1 + 2 + 3 + 16 + 0 + 27 + 4 + 115 + 384 + 11 + 40 = 603 \implies A+B+C+D+E+F+G+H+I+J+K = 1+2+3+16+0+27+4+115+384+11+40=\boxed{603}

Please complete the solution. Been waiting a long time.

Digvijay Singh - 3 years, 7 months ago

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The rest are very long.

Chew-Seong Cheong - 3 years, 7 months ago

I have given all the answers. The solutions are simply too long.

Chew-Seong Cheong - 3 years, 7 months ago

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