Evaluate:
∫ 0 π / 2 sin x cos x tan x ln ( tan x ) d x
The answer can be expressed as a π ( b − π ) c , where a , b , and c are positive integers and c is square-free; find 1 0 0 a + 1 0 b + c .
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Use the u -sub, u = tan x → d u = 2 tan x sec 2 x d x :
I = ∫ 0 π / 2 sin x cos x tan x ln ( tan x ) d x = 4 ∫ 0 π / 2 2 sec 4 x tan x sec 2 x tan 2 x ln ( tan x ) d x = 4 ∫ 0 ∞ ( u 4 + 1 ) 2 u 4 ln u d u
This integral can be solved using complex integration, with a keyhole contour; but a simpler, strictly-real solution introduces the integral:
F ( a ) = ∫ 0 ∞ x a + 1 d x = ∫ 0 ∞ ∫ 0 ∞ e − t e − t x a d t d x = a 1 Γ ( a 1 ) Γ ( 1 − a 1 ) = a π csc ( a π )
Then, by Feynman's trick of 'differentiation under the integral sign':
F ′ ( a ) = d a d ∫ 0 ∞ x a + 1 d x = ∫ 0 ∞ ( x a + 1 ) 2 x a ln x d x = a 1 ( a π csc ( a π ) ) ( 1 − a π cot ( a π ) )
Thus, the integral is:
I = 4 F ′ ( 4 ) = 4 ( 4 1 ( 4 π csc ( 4 π ) ) ( 1 − 4 π cot ( 4 π ) ) ) = 1 6 1 π ( 4 − π ) 2
and the answer is 1 0 0 ⋅ 1 6 + 1 0 ⋅ 4 + 2 = 1 6 4 2 .
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The substitution t = tan 2 x gives I = ∫ 0 2 1 π sin x cos x tan x ln ( tan x ) d x = 4 1 ∫ 0 ∞ ( t + 1 ) 2 t 4 1 ln t d t and hence I = 4 1 ( ∂ a ∂ B ( a , b ) − ∂ b ∂ B ( a , b ) ) ∣ ∣ ∣ a = 4 5 , b = 4 3 = 4 1 B ( a , b ) ( ψ ( a ) − ψ ( b ) ) ∣ ∣ ∣ a = 4 5 , b = 4 3 = 4 1 B ( 4 5 , 4 3 ) [ ψ ( 4 5 ) − ψ ( 4 3 ) ] = 4 Γ ( 2 ) Γ ( 4 5 ) Γ ( 4 3 ) [ 4 + ψ ( 4 1 ) − ψ ( 4 3 ) ] = 1 6 Γ ( 4 1 ) Γ ( 4 3 ) [ 4 + ψ ( 4 1 ) − ψ ( 4 1 ) − π ] = 1 6 π 2 ( π − 4 ) = 1 6 π ( 4 − π ) 2 making the answer 1 0 0 × 1 6 + 1 0 × 4 + 2 = 1 6 4 2 .