巧算有理数

问题描述:

巧算有理数
1-2+3-4+...+2007-2008和69+699+6999+69999+699999.

1-2+3-4+...+2007-2008
=(1-2)+(3-4)+...+(2007-2008)
=-1-1-……-1
=-1004
69+699+6999+69999+699999.
=(70-1)+(700-1)+……+(700000-1)
=777770-5
=777765