∫ 0 2 π 1 0 0 { tan ( x ) } { cot ( x ) } d x
Find the value of the closed form of the above integral. Round off your answer to the nearest integer.
Notation:
{
⋅
}
denotes the
fractional part function
.
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How did you solve last step i.e. getting sinhπ term?
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It is a well-known fact that π z sin π z = n = 1 ∏ ∞ ( 1 − n 2 z 2 ) a fact that is most easily proved by contour integration. Now put z = i into this formula.
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Thank you.BTW is there any special name to it
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Note that f ( x ) = 1 0 0 { tan x } { cot x } is symmetric over [ 0 , 2 1 π ] , and hence I = ∫ 0 2 1 π f ( x ) d x = 2 ∫ 4 1 π 2 1 π f ( x ) d x = 2 0 0 ∫ 4 1 π 2 1 π { tan x } cot x d x = 2 0 0 ∫ 4 1 π 2 1 π ( tan x − ⌊ tan x ⌋ ) cot x d x = 5 0 π − 2 0 0 ∫ 4 1 π 2 1 π ⌊ tan x ⌋ cot x d x = 5 0 π − 2 0 0 n = 1 ∑ ∞ n ∫ tan − 1 n tan − 1 ( n + 1 ) cot x d x = 5 0 π − 2 0 0 n = 1 ∑ ∞ n [ ln ( sin x ) ] tan − 1 n tan − 1 ( n + 1 ) = 5 0 π − 2 0 0 N → ∞ lim n = 1 ∑ N n [ ln ( 1 + ( n + 1 ) 2 n + 1 ) − ln ( 1 + n 2 n ) ] = 5 0 π − 2 0 0 N → ∞ lim { n = 2 ∑ N + 1 ( n − 1 ) ln ( 1 + n 2 n ) − n = 1 ∑ N n ln ( 1 + n 2 n ) } = 5 0 π − 2 0 0 N → ∞ lim { N ln ( 1 + ( N + 1 ) 2 N + 1 ) − n = 1 ∑ N ln ( 1 + n − 2 1 ) } = 5 0 π + 1 0 0 N → ∞ lim N ln ( 1 + ( N + 1 ) − 2 ) − 1 0 0 N → ∞ lim ln ( n = 1 ∏ N ( 1 + n − 2 ) ) = 5 0 π − 1 0 0 ln ( n = 1 ∏ ∞ ( 1 + n − 2 ) ) = 5 0 π − 1 0 0 ln ( π sinh π ) = 2 6 . 8 9 4 9 9 2 8 2 . . . making the answer 2 7 .