1-2+3-4+5-6+......+1997-1998+1999

原式=(1-2)+(3-4)+(5-6)+……+(1997-1998)+1999
=(-1)×(1998÷2)+1999------------(到2有一个-1,到4有两个-1,到2n有n个-1,所以到1998有1998÷2个-1)
=-999+1999
=1000

等差数列问题
1-2+3-4+5-6+......+1997-1998+1999
=(1-2)+(3-4)+(5-6)......+(1997-1998)+1999
=(1998/2)*(-1)+1999
=999*(-1)+1999
=-999+1999
=1000

1+1999=2000,2+1998=2000也次类推有999组乘以20001998000然后发先中间多个55再加上55答案是200001998055

1-2+3-4+5-6+......+1997-1998+1999
=(1+1999)-(2+1998)+(3+1997)......-1000
=-1000

1-2+3-4+5-6+......+1997-1998+1999
=(1-2)+(3-4)+(5-6)......+(1997-1998)+1999
=(1998/2)*(-1)+1999
=999*(-1)+1999
=-999+1999
=1000