Paneer Definite Masala 7

Calculus Level 5

1 ( 1 + 5 ) / 2 x 2 + 1 x 4 x 2 + 1 ln ( 1 + x 1 x ) d x \large \int_1^{(1+\sqrt5)/2} \dfrac{x^2+1}{x^4-x^2+1} \ln \left( 1 + x - \dfrac1x\right) \, dx

If the value of the integral above is equal to a π b c ln d \dfrac {a\pi^b}c \ln d , where a , b , c a,b,c and d d are positive integers with d d minimized, find a + b + 2 c + d a+b+2c+d .


The answer is 20.

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1 solution

Mark Hennings
Jul 31, 2016

Putting y = x x 1 y = x - x^{-1} , this integral becomes I = 0 1 ln ( 1 + y ) y 2 + 1 d y I \; = \; \int_0^1 \frac{\ln(1+y)}{y^2+1}\,dy Putting y = tan θ y = \tan\theta , we obtain I = 0 1 4 π ln ( 1 + tan θ ) d θ I \;=\; \int_0^{\frac14\pi}\ln(1 + \tan\theta)\,d\theta Putting θ = 1 4 π ϕ \theta = \tfrac14\pi - \phi we obtain I = 0 1 4 π ln ( 2 1 + tan ϕ ) d ϕ = 1 4 π ln 2 I = 1 8 π ln 2 I \; = \; \int_0^{\frac14\pi} \ln\left(\frac{2}{1 + \tan\phi}\right)\,d\phi \; = \; \tfrac14\pi\ln2 - I \; =\; \tfrac18\pi\ln2 making the answer 1 + 1 + 2 × 8 + 2 = 20 1 + 1 + 2\times8 + 2 = \boxed{20} .

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