1-2+3-4+5-6+7.-2004+2005
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![1-2+3-4+5-6+7.-2004+2005](/uploads/image/z/443333-29-3.jpg?t=1-2%2B3-4%2B5-6%2B7.-2004%2B2005)
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1-2+3-4+5-6+7.-2004+2005
1-2+3-4+5-6+7.-2004+2005
1-2+3-4+5-6+7.-2004+2005
(1-2)+(3-4)+(5-6)+(7-8)+.+(2003-2004)+2005
等差数列分项求和
1+(-2+3)+(-4+5)+……+(-2004+2005)=1*1003=1003
=(1-2)+(3-4)+(5-6)+......+(2003-2004)+2005
=-1×(2004╱2)+2005
=1003