1-2+3-4+5-6+…+2007-2008+2009.
问题描述:
1-2+3-4+5-6+…+2007-2008+2009.
答
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)÷2+1]÷2},
=1010025-1009020,
=1005.