∫ 0 2 π tan x x d x + ∫ 0 2 π ln sin x d x + ∫ 4 π 2 π ln tan x d x ∫ 0 2 π sin x x d x + ∫ 0 2 π ln ( 1 + tan x ) d x − ∫ 0 4 π ln ( ( 1 + tan x ) 2 ) d x − ∫ 0 4 π ln tan x d x = ?
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Let I = ∫ 0 2 π tan x x d x + ∫ 0 2 π ln ( sin x ) d x + ∫ 4 π 2 π ln ( tan x ) d x ∫ 0 2 π sin x x d x + ∫ 0 2 π ln ( 1 + tan x ) d x − ∫ 0 4 π ln ( ( 1 + tan x ) 2 ) d x − ∫ 0 4 π ln ( tan x ) d x = I 5 + I 6 + I 7 I 1 + I 2 − I 3 − I 4
I 3 = ∫ 0 4 π ln ( ( 1 + tan x ) 2 ) d x = 2 ∫ 0 4 π ln ( 1 + tan x ) d x
I 2 = ∫ 0 2 π ln ( 1 + tan x ) d x = ∫ 4 π 2 π ln ( 1 + tan x ) d x + ∫ 0 4 π ln ( 1 + tan x ) d x = ∫ 4 π 2 π ln ( 1 + tan ( 4 3 π − x ) ) d x + 2 1 I 3 = ∫ 4 π 2 π ln ( 1 − tan ( x + 4 π ) ) d x + 2 1 I 3 = ∫ 0 4 π ln ( 1 − tan ( u + 2 π ) ) d u + 2 1 I 3 = ∫ 0 4 π ln ( 1 + tan ( 2 π − u ) ) d u + 2 1 I 3 = ∫ 0 4 π ln ( 1 + cot u ) d u + 2 1 I 3 = ∫ 0 4 π ln ( tan u tan u + 1 ) d u + 2 1 I 3 = ∫ 0 4 π ln ( 1 + tan u ) d u − ∫ 0 4 π ln ( tan u ) d u + 2 1 I 3 = I 3 − I 4 Using identity: ∫ a b f ( x ) d x = ∫ a b f ( a + b − x ) d x Note that tan ( π − θ ) = − tan θ Let u = x − 4 π ⟹ d u = d x Note that tan ( π − θ ) = − tan θ
I 4 = ∫ 0 4 π ln ( tan x ) d x = x ln ( tan x ) ∣ ∣ ∣ ∣ 0 4 π − ∫ 0 4 π tan x x sec 2 x d x = 0 − x → 0 lim x ln ( tan x ) − ∫ 0 4 π sin x cos x x d x = x → 0 lim tan x x 2 sec x − 2 1 ∫ 0 4 π sin 2 x x d x = x → 0 lim sin x cos x x 2 − 2 1 ∫ 0 2 π sin u u d u = x → 0 lim cos x x − 2 1 I 1 = 0 − 2 1 I 1 By integration by parts by L’H o ˆ pital’s rule Let u = 2 x ⟹ d u = 2 d x
I 1 = − 2 I 4
I 6 = ∫ 0 2 π ln ( sin x ) d x = x ln ( sin x ) ∣ ∣ ∣ ∣ 0 2 π − ∫ 0 2 π sin x x cos x d x = 0 − ∫ 0 2 π tan x x d x = − I 5 By integration by parts
I 7 = ∫ 4 π 2 π ln ( tan x ) d x = ∫ 0 2 π ln ( tan x ) d x − ∫ 0 4 π ln ( tan x ) d x = 2 1 ∫ 0 2 π [ ln ( tan x ) + ln ( tan ( 2 π − x ) ) ] d x − I 4 = 2 1 ∫ 0 2 π [ ln ( tan x ) + ln ( cot x ) ] d x − I 4 = 0 − I 4 Using identity: ∫ a b f ( x ) d x = ∫ a b f ( a + b − x ) d x
Therefore, we have
I = I 5 + I 6 + I 7 I 1 + I 2 − I 3 − I 4 = I 5 − I 5 − I 4 − 2 I 4 + I 3 − I 4 − I 3 − I 4 = − I 4 − 4 I 4 = 4