1+4-7+10-……+(-1)^n(3n+2) 求前N项和
问题描述：
1+4-7+10-……+(-1)^n(3n+2) 求前N项和
答
若n=2m+1(意思是n为奇数）时,原式可化为：=-1-7-13-…………-(3n+2) +4+10+16……+(3n+2-6) =-[1+7+13+……+(3n+2)]+[4+10+16……+(3n-4)] =-[(1+(3n+2]*(n+1)/2/2+(4+3n-4)*(n-1)/2/2 =-(3n+3)(n+1)/4+3n(n-1)/4 =-...