Evaluate ∫ − 1 1 y → ∞ lim ( arctan ( z = 1 ∑ y x 2 x ln ( y − z + 1 ) ) + arctan ( ∑ z = 1 y x 2 x ln ( y − z + 1 ) 1 ) ) d x
Note: The definition of arctan ( x ) used in this problem is restricted to the range ( − 2 π , 2 π ) .
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From easy trigonometry in a right angled triangle you can see that
a r c t a n ( a ) + a r c t a n ( a 1 ) = 2 π
The y limit is seen to be irrelevant and the integral then simplifies to
∫ − 1 1 2 π d x = π
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I = ∫ − 1 1 y → ∞ lim ( tan − 1 ( z = 1 ∑ y x 2 x ln ( y − z + 1 ) ) + tan − 1 ( ∑ z = 1 y x 2 x ln ( y − z + 1 ) 1 ) ) d x = ∫ − 1 1 y → ∞ lim ( tan − 1 ( z = 1 ∑ y x 2 x ln ( y − z + 1 ) ) + 2 π − tan − 1 ( z = 1 ∑ y x 2 x ln ( y − z + 1 ) ) ) d x = ∫ − 1 1 2 π d x = 2 π x ∣ ∣ ∣ ∣ − 1 1 = π ≈ 3 . 1 4 2