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1-2+3-4+5-6+…+2007-2008+2009=(1+3+5+…+2009)-(2+3+4+…2008)={((1+2009)×[(2009-1)÷2+1]÷2}-{(2+2008)×(2008)÷2+1]÷2}、=1010025-1090、=1005.
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