Integral Integral please please

Calculus Level pending

0 1 ( π 2 π 2 log ( 1 a + sin ( x ) ) d x ) d a = 1 2 π ( π log ( b ) ) \int_0^1 \left(\int_{-\frac{\pi }{2}}^{\frac{\pi }{2}} \log \left(\frac{1}{a}+\sin (x)\right) \, dx\right) \, da=\frac{1}{2} \pi (\pi -\log (b))

Submit b b .


The answer is 4.

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1 solution

Mark Hennings
Dec 24, 2017

If we define F ( b ) = 1 2 π 1 2 π ln ( b + sin x ) d x = 1 2 π π ln ( b + sin x ) d x = 1 2 π π ln ( b + cos x ) d x F(b) \; = \; \int_{-\frac12\pi}^{\frac12\pi}\ln(b + \sin x)\,dx \; = \; \tfrac12\int_{-\pi}^\pi \ln(b + \sin x)\,dx \; = \; \tfrac12\int_{-\pi}^\pi \ln(b + \cos x)\,dx for b 1 b \ge 1 , then F ( 1 ) = 1 2 π π ln ( 2 cos 2 1 2 x ) d x = 0 π ln ( 2 cos 2 1 2 x ) d x = 2 0 1 2 π ln ( 2 cos 2 x ) d x = π ln 2 + 4 0 1 2 π ln ( cos x ) d x = π ln 2 + B α α = β = 1 2 = π ln 2 + B ( 1 2 , 1 2 ) [ ψ ( 1 2 ) ψ ( 1 ) ] = π ln 2 \begin{aligned} F(1) & = \; \tfrac12\int_{-\pi}^\pi \ln(2\cos^2\tfrac12x)\,dx \; = \; \int_0^\pi \ln(2\cos^2\tfrac12x)\,dx \; = \; 2\int_0^{\frac12\pi}\ln(2\cos^2x)\,dx \\ & = \; \pi\ln2 + 4\int_0^{\frac12\pi}\ln(\cos x)\,dx \; = \; \pi\ln2 + \left.\frac{\partial B}{\partial \alpha}\right|_{\alpha =\beta=\frac12} \\ & = \; \pi\ln2 + B(\tfrac12,\tfrac12)\big[\psi(\tfrac12)-\psi(1)\big] \; = \; -\pi\ln2 \end{aligned} while F ( b ) = 1 2 π π d x b + cos x = z = 1 1 2 b + z + z 1 d z i z = i z = 1 d z z 2 + 2 b z + 1 = i z = 1 d z ( z u + ) ( z u ) = 2 π R e s z = u + 1 ( z u + ) ( z u ) = 2 π u + u = π b 2 1 \begin{aligned} F'(b) & = \; \tfrac12\int_{-\pi}^\pi \frac{dx}{b + \cos x} \; = \; \int_{|z|=1}\frac{1}{2b + z + z^{-1}} \frac{dz}{iz} \; = \; -i\int_{|z|=1}\frac{dz}{z^2 + 2bz + 1} \\ & = \; -i\int_{|z|=1} \frac{dz}{(z - u_+)(z - u_-)} \; = \; 2\pi\,\mathrm{Res}_{z=u_+} \frac{1}{(z-u_+)(z-u_-)} \;= \; \frac{2\pi}{u_+ - u_-} \;= \; \frac{\pi}{\sqrt{b^2-1}} \end{aligned} for b > 1 b > 1 , where u ± = b ± b 2 1 u_\pm = -b \pm\sqrt{b^2-1} . The desired double integral is 0 1 ( 1 2 π 1 2 π ln ( a 1 + sin x ) d x ) d a = 0 1 F ( a 1 ) d a = 1 F ( b ) b 2 d b = [ F ( b ) b ] 1 + 1 F ( b ) b d b = π 1 d b b b 2 1 + F ( 1 ) = π 0 1 d a 1 a 2 + F ( 1 ) = 1 2 π 2 + F ( 1 ) = 1 2 π ( π ln 4 ) \begin{aligned} \int_0^1 \left(\int_{-\frac12\pi}^{\frac12\pi}\ln(a^{-1} + \sin x)\,dx\right)\,da & = \; \int_0^1 F\big(a^{-1}\big)\,da \; = \; \int_1^\infty \frac{F(b)}{b^2}\,db \\ & = \; \left[-\frac{F(b)}{b}\right]_1^\infty + \int_1^\infty \frac{F'(b)}{b}\,db \; = \; \pi\int_1^\infty \frac{db}{b\sqrt{b^2-1}} + F(1) \\ & = \; \pi\int_0^1 \frac{da}{\sqrt{1-a^2}} + F(1) \; = \; \tfrac12\pi^2 + F(1) \\ & = \; \tfrac12\pi(\pi - \ln4) \end{aligned} making the answer 4 \boxed{4} .

π \pi is not period of s i n x sinx then how 1/2 is coming out ?

Kushal Bose - 3 years, 5 months ago

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The integral of ln ( b + sin x ) \ln(b+\sin x) from 1 2 π \tfrac12\pi to π \pi is the same as the integral from 0 0 to 1 2 π \tfrac12\pi , and the integral from π -\pi to 1 2 π -\tfrac12\pi is equal to the integral from 1 2 π -\tfrac12\pi to 0 0 . Thus the integral from 1 2 π -\tfrac12\pi to 1 2 π \tfrac12\pi is half the integral from π -\pi to π \pi .

Mark Hennings - 3 years, 5 months ago

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Ok Thanks....

Kushal Bose - 3 years, 5 months ago

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