-1+3-5+7-9+11-.-1997+1999

题目

-1+3-5+7-9+11-.-1997+1999

答案

(-1+3)+(-5+7)+(-9+11)+……+(-1997+1999) (以两两分组拆开)
=2+2+2+2+..+2
=2[(1999+1)÷2÷2] (有500组)
=500×2
=1000