$\text { Đáp án: }$
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` 1-2-3+4+5+7+8+...+1997-1998-1999+2000+2100 `
` =(1-2-3+4)+(5-7-8+9)+...+(1997-1998-1999+2000)+2001 `
` =0+0+...+0+2001 `
` =2001 `
` 1-3+5-7+...+2001-2003+2005 `
` =(1-3)+(5-7)+...+(2001-2003)+2005 `
` =-2+(-2)+...+(-2)+2005 `
` =-2.1002+2005 `
` =-2004+2005 `
` =1 `
` 942-2567+2563-1942 `
` =(942-1942)-(2563-2567) `
` =-1000-4 `
` =-1004 `