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Calculus Level 5

0 e x 8 cos ( x 8 ) x 3 ln ( x ) d x \large \displaystyle \int_{0}^{\infty}e^{-x^{8}}\cos(x^{8})x^{3}\ln(x)\, dx If the integral above can be expressed as π 1 A B [ A + C ( γ C + D A ln ( A ) ) + π C E A C ] \dfrac{-\pi^{\frac{1}{A}}}{B}\left[ \sqrt{\sqrt{A}+C}\left(\gamma^{C}+\dfrac{D}{A}\ln(A)\right)+\dfrac{\pi^{C}}{E}\sqrt{\sqrt{A}-C}\right]

with positive integers A , B , C , D , E A,B,C,D,E and all fractions are irreducible and A A being square free, then evaluate A + B + C + D + E A+B+C+D+E .

Notation: γ 0.5772 \gamma \approx 0.5772 denotes the Euler-Mascheroni constant .


The answer is 140.

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2 solutions

Mark Hennings
Nov 23, 2016

The substitution u = x 8 u = x^8 makes the integral I = 1 64 0 e u cos u u 1 2 ln u d u = 1 64 A ( 1 2 ) I \; = \; \tfrac{1}{64} \int_0^\infty e^{-u} \,\cos u \,u^{-\frac12} \, \ln u\,du \; = \; \tfrac{1}{64}A'(\tfrac12) where A A is the function A ( a ) = 0 e u cos u u a 1 d u = 2 1 2 a cos π a 4 Γ ( a ) a > 0 . A(a) \; = \; \int_0^\infty e^{-u} \,\cos u\, u^{a-1}\,du \; = \; 2^{-\frac12a} \,\cos\tfrac{\pi a}{4}\, \Gamma(a) \hspace{1cm} a > 0 \;. Differentiating and simplifying, we obtain A ( 1 2 ) = π 2 5 4 cos π 8 ln 2 + π 2 1 4 cos π 8 ψ ( 1 2 ) π π 2 17 4 sin π 8 I = π 128 [ 2 + 1 ( γ + 5 2 ln 2 ) + 1 4 π 2 1 ] \begin{array}{rcl} A'(\tfrac12) & = & \displaystyle -\frac{\sqrt{\pi}}{2^{\frac54}} \cos \tfrac{\pi}{8}\, \ln 2 + \frac{\sqrt{\pi}}{2^{\frac14}} \cos \tfrac{\pi}{8}\, \psi(\tfrac12) -\frac{\pi\sqrt{\pi}}{2^{\frac{17}{4}}}\sin\tfrac{\pi}{8} \\ I & = & \displaystyle-\frac{\sqrt{\pi}}{128}\left[ \sqrt{\sqrt{2}+1}\left(\gamma + \tfrac52\ln2\right) + \tfrac14\pi\sqrt{\sqrt{2}-1}\right] \end{array} making the answer 2 + 128 + 1 + 5 + 4 = 140 2 + 128 + 1 + 5 + 4 = \boxed{140} .

Kunal Gupta
Nov 23, 2016

The answer is: π 1 2 128 [ 2 + 1 ( γ + 5 2 ln ( 2 ) ) + π 4 2 1 ] \dfrac{-\pi^{\frac{1}{2}}}{128}\left[ \sqrt{\sqrt{2}+1}\left(\gamma+\dfrac{5}{2}\ln(2)\right)+\dfrac{\pi}{4}\sqrt{\sqrt{2}-1}\right] Will post the solution later

your problems are good! is this original?

Usha Gupta - 4 years, 6 months ago

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yes, I created it at around 4 AM , I wasn't feeling sleepy!

Kunal Gupta - 4 years, 6 months ago

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