Some Calculus!

Calculus Level 5

0 π x 2 5 2 cos x d x \int\limits_0^\pi{\frac{{{x^2}}}{{\sqrt 5-2\cos x}}}\operatorname d\!x

If the solution of the above integral can be depicted as:

π a b + 2 π ln c ( d + e f ) \frac{{{\pi^a}}}{{b}}+2\pi \ln^c \left({\frac{{d+\sqrt e }}{f}}\right)

where a , b , c , d , e , f a,b,c,d,e,f are integers, what is the value of a + b + c d + e f a+b+c-d+e-f ?


The answer is 22.

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2 solutions

Mark Hennings
Jan 4, 2017

My solution is similar, but not identical.

We have 0 1 ln x x + a d x = 1 6 π 2 1 2 ln 2 a + 1 2 ln 2 ( 1 + a ) + L i 2 ( a 1 + a ) a > 0 \int_0^1 \frac{\ln x}{x+a}\,dx \; = \; -\tfrac16\pi^2 - \tfrac12\ln^2a + \tfrac12\ln^2(1+a) + \mathrm{Li}_2\left(\tfrac{a}{1+a}\right) \hspace{1cm} a > 0 and hence, with ϕ = 1 2 ( 5 + 1 ) \phi = \tfrac12(\sqrt{5}+1) being the Golden Section, 0 1 ln x x + ϕ d x = 1 6 π 2 + 3 2 ln 2 ϕ + L i 2 ( ϕ 1 ) 0 1 ln x x + ϕ 1 d x = 1 6 π 2 + L i 2 ( ϕ 2 ) \begin{aligned} \int_0^1 \frac{\ln x}{x+\phi}\,dx & = -\tfrac16\pi^2 + \tfrac32\ln^2\phi + \mathrm{Li}_2(\phi^{-1}) \\ \int_0^1 \frac{\ln x}{x + \phi^{-1}}\,dx & = -\tfrac16\pi^2 + \mathrm{Li}_2(\phi^{-2}) \end{aligned} and hence 0 1 ln x x 2 + 5 x + 1 d x = 0 1 ln x ( x + ϕ ) ( x + ϕ 1 ) d x = L i 2 ( ϕ 2 ) L i 2 ( ϕ 1 ) 3 2 ln 2 ϕ = ( 1 15 π 2 1 4 ln 2 ( ϕ 2 ) ) ( 1 10 π 2 ln 2 ( ϕ 1 ) ) 3 2 ln 2 ϕ = 1 30 π 2 3 2 ln 2 ϕ \begin{aligned} \int_0^1 \frac{\ln x}{x^2 + \sqrt{5}x + 1}\,dx & = \int_0^1 \frac{\ln x}{(x+\phi)(x+\phi^{-1})}\,dx \; = \; \mathrm{Li}_2(\phi^{-2}) - \mathrm{Li}_2(\phi^{-1}) - \tfrac32\ln^2\phi \\ & = \left(\tfrac{1}{15}\pi^2 - \tfrac14\ln^2(\phi^{-2})\right) - \left(\tfrac{1}{10}\pi^2 - \ln^2(\phi^{-1})\right) - \tfrac32\ln^2\phi \\ & = -\tfrac{1}{30}\pi^2 - \tfrac32\ln^2\phi \end{aligned} using standard properties of the dilogarithm function L i 2 \mathrm{Li}_2 .

Now consider the cut plane with the negative real axis removed, and the key-hole contour Γ = γ 1 γ 2 γ 3 + γ 4 \Gamma = \gamma_1 - \gamma_2 - \gamma_3 + \gamma_4 , where

  • γ 1 \gamma_1 is the (almost) circular arc z = e i x z = e^{ix} for π < x < π -\pi < x < \pi ,
  • γ 2 \gamma_2 is the straight-line segment z = x e π i z = x e^{\pi i} for ϵ < x < 1 \epsilon < x < 1 , just above the cut,
  • γ 3 \gamma_3 is the (almost) circular arc z = ϵ e i x z = \epsilon e^{ix} for π < x < π -\pi < x < \pi ,
  • γ 4 \gamma_4 is the straight-line segment z = x e π i z = x e^{-\pi i} for ϵ < x < 1 \epsilon < x < 1 , just below the cut.

The function f ( z ) = ( log z ) 2 z 2 5 z + 1 f(z) \; = \; \frac{(\log z)^2}{z^2 - \sqrt{5}z + 1} is analytic inside and on the curve Γ \Gamma , except for a simple pole at z = ϕ 1 z = \phi^{-1} . Now γ 1 f ( z ) d z = π π ( i x ) 2 i e i x d x e i x ( 2 cos x 5 ) = 2 i 0 1 x 2 d x 5 2 cos x ( γ 2 γ 4 ) f ( z ) d z = 0 1 ( ln x + π i ) 2 ( ln x π i ) 2 x 2 + 5 x + 1 d x = 4 π i 0 1 ln x x 2 + 5 x + 1 d x γ 3 f ( z ) d z = O ( ϵ ln ϵ ) ϵ 0 \begin{aligned} \int_{\gamma_1} f(z)\,dz & = \int_{-\pi}^\pi \frac{(ix)^2 i e^{ix}\,dx}{e^{ix}(2\cos x - \sqrt{5})} \; = \; 2i\int_0^1 \frac{x^2\,dx}{\sqrt{5} - 2\cos x} \\ \left(\int_{\gamma_2} - \int_{\gamma_4}\right)f(z)\,dz & = -\int_0^1 \frac{(\ln x + \pi i)^2 - (\ln x - \pi i)^2}{x^2 + \sqrt{5}x + 1}\,dx \\ & = -4\pi i\int_0^1 \frac{\ln x}{x^2 + \sqrt{5}x + 1}\,dx \\ \int_{\gamma_3}f(z)\,dz & = O\big(\epsilon \ln\epsilon) \hspace{1cm} \epsilon \to 0 \end{aligned} Thus, since ( γ 1 γ 2 γ 3 + γ 4 ) f ( z ) d z = 2 π i R e s z = ϕ 1 f ( z ) = 2 π i ln 2 ( ϕ 1 ) = 2 π i ln 2 ϕ \left(\int_{\gamma_1} - \int_{\gamma_2} - \int_{\gamma_3} + \int_{\gamma_4}\right)f(z)\,dz \; = \; 2\pi i \mathrm{Res}_{z=\phi^{_1}} f(z) \; = \; -2\pi i \ln^2(\phi^{-1}) \; = \; -2\pi i \ln^2\phi we deduce, letting ϵ 0 \epsilon \to 0 , that 0 π x 2 d x 5 2 cos x + 2 π 0 1 ln x x 2 + 5 x + 1 d x = π ln 2 ( ϕ 1 ) = π ln 2 ϕ \int_0^\pi \frac{x^2\,dx}{\sqrt{5} - 2\cos x} + 2\pi\int_0^1\frac{\ln x}{x^2 + \sqrt{5}x + 1}\,dx \; = \; -\pi\ln^2(\phi^{-1}) \; = \; -\pi\ln^2\phi and hence 0 π x 2 d x 5 2 cos x = 1 15 π 3 + 2 π ln 2 ϕ \int_0^\pi \frac{x^2\,dx}{\sqrt{5} - 2\cos x} \; = \; \tfrac{1}{15}\pi^3 + 2\pi\ln^2\phi which makes the answer 3 + 15 + 2 1 + 5 2 = 22 3 + 15 + 2 - 1 + 5 - 2 = \boxed{22} .

Pranav Saxena
Jan 1, 2017

Obviously this is not an original question.(Wish i had that kind of brains)

Credit goes to china(tian27546).

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