∫ 0 1 ( ∫ − 2 π 2 π lo g ( a 1 + sin ( x ) ) d x ) d a = 2 1 π ( π − lo g ( b ) )
Submit b .
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π is not period of s i n x then how 1/2 is coming out ?
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The integral of ln ( b + sin x ) from 2 1 π to π is the same as the integral from 0 to 2 1 π , and the integral from − π to − 2 1 π is equal to the integral from − 2 1 π to 0 . Thus the integral from − 2 1 π to 2 1 π is half the integral from − π to π .
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If we define F ( b ) = ∫ − 2 1 π 2 1 π ln ( b + sin x ) d x = 2 1 ∫ − π π ln ( b + sin x ) d x = 2 1 ∫ − π π ln ( b + cos x ) d x for b ≥ 1 , then F ( 1 ) = 2 1 ∫ − π π ln ( 2 cos 2 2 1 x ) d x = ∫ 0 π ln ( 2 cos 2 2 1 x ) d x = 2 ∫ 0 2 1 π ln ( 2 cos 2 x ) d x = π ln 2 + 4 ∫ 0 2 1 π ln ( cos x ) d x = π ln 2 + ∂ α ∂ B ∣ ∣ ∣ ∣ α = β = 2 1 = π ln 2 + B ( 2 1 , 2 1 ) [ ψ ( 2 1 ) − ψ ( 1 ) ] = − π ln 2 while F ′ ( b ) = 2 1 ∫ − π π b + cos x d x = ∫ ∣ z ∣ = 1 2 b + z + z − 1 1 i z d z = − i ∫ ∣ z ∣ = 1 z 2 + 2 b z + 1 d z = − i ∫ ∣ z ∣ = 1 ( z − u + ) ( z − u − ) d z = 2 π R e s z = u + ( z − u + ) ( z − u − ) 1 = u + − u − 2 π = b 2 − 1 π for b > 1 , where u ± = − b ± b 2 − 1 . The desired double integral is ∫ 0 1 ( ∫ − 2 1 π 2 1 π ln ( a − 1 + sin x ) d x ) d a = ∫ 0 1 F ( a − 1 ) d a = ∫ 1 ∞ b 2 F ( b ) d b = [ − b F ( b ) ] 1 ∞ + ∫ 1 ∞ b F ′ ( b ) d b = π ∫ 1 ∞ b b 2 − 1 d b + F ( 1 ) = π ∫ 0 1 1 − a 2 d a + F ( 1 ) = 2 1 π 2 + F ( 1 ) = 2 1 π ( π − ln 4 ) making the answer 4 .