Find .
Note: The double factorial is defined as
For an even positive integer,
For an odd positive integer,
If ,
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The sum is n = 0 ∑ 1 7 2 9 4 n + 2 ( − 1 ) n ( n 1 7 2 9 ) = = = n = 0 ∑ 1 7 2 9 ( − 1 ) n ( n 1 7 2 9 ) ∫ 0 1 x 4 n + 1 d x = ∫ 0 1 ( 1 − x 4 ) 1 7 2 9 x d x 2 1 ∫ 0 1 ( 1 − y 2 ) 1 7 2 9 d y = 2 1 ∫ 0 2 1 π cos 3 4 5 9 θ d θ 2 1 × 3 4 5 9 ! ! 2 1 7 2 9 1 7 2 9 ! = 3 4 5 9 ! ! 2 1 7 2 8 1 7 2 9 ! using the substitutions y = x 2 and y = sin θ . This makes the answer 1 7 2 8 + 1 7 2 9 + 3 4 5 9 = 6 9 1 6 .