n = 1 ∑ ∞ 1 6 n 2 − 1 6 n + 3 8 = ?
Give your answer to 2 decimal places.
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It's better to mention that you perform partial fraction. For clarity, can you explain why 4 π = 1 1 − 3 1 + 5 1 − 7 1 + … ?
@ChallengeMaster, Edited! :)
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n = 1 ∑ ∞ 1 6 n 2 − 1 6 n + 3 8 = n = 1 ∑ ∞ ( 4 n − 1 ) ( 4 n − 3 ) 8
By turning it into partial fractions, we obtain:
= 4 n = 1 ∑ ∞ ( 4 n − 3 1 − 4 n − 1 1 )
= 4 ( 1 1 − 3 1 + 5 1 − 7 1 + 9 1 − … ad inf )
We know by Gregory's Series that:
tan − 1 ( x ) = x − 3 x 3 + 5 x 5 − 7 x 7 + … ad inf
Therefore, putting x = 1 in the above series, we obtain the next step of solution as
= 4 tan − 1 ( 1 ) = 4 ( 4 π ) = π = 3 . 1 4