11×3+13×5+15×7+… +12009×2011.
The original formula is 12 × (1-13 + 13-15 + 15-17 +...) +12009-12011)=12×(1-12011)=10052011.
How to add 1 / 3x3 + 1 / 3x5 + 1 / 5x7 + 1 / 7x9 to 1 / 2009x2011,
1 / n*(n+2)=1/2(1/n-1/(n+2))
Original formula = 1 / 2 * (1-1 / 3 + 1 / 3-1 / 5 + ellipsis + 1 / 2009-1 / 2011) = 1 / 2 (1-1 / 2011) = 1005 / 2011
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