Let Sn = - 1 + 3-5 + 7 - +(- 1) n (2n-1), then SN=______ .
When n is even, Sn = (- 1 + 3) + (- 5 + 7) + +[-(2n-3)+(2n-1)]=2+2+… +When n is odd, Sn = (- 1 + 3) + (- 5 + 7) + +[-(2n-5)+(2n-3)]+(-1)×(2n-1)=2+2+… +2 (total N − 12 items) - (2n-1) = 2 × (n
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