1-2+3-4+5-6.+（-1）的n+1次方乘n
=(1-2)+(3-4)+(5-6)+.+（-1）的n+1次方乘n
=-1-1+...+(-1)^(n+1)*n
n为偶数时,值=-n/2
n为奇数时,值=n-(n-1)/2
一减二加三减四加五减六.加负一的N加一次方乘N得多少
1-2+3-4+5-6.+（-1）的n+1次方乘n
=(1-2)+(3-4)+(5-6)+.+（-1）的n+1次方乘n
=-1-1+...+(-1)^(n+1)*n
n为偶数时,值=-n/2
n为奇数时,值=n-(n-1)/2