n为偶数时
-1+4-7+10-.-(3n-5)+(3n-2)
=(-1+4)+(-7+10)+...+[-(3n-5)+(3n-2)]
=3×n/2
=3n/2
n为奇数时
-1+4-7+10-13+...+(3n-5)-(3n-2)
=-1+(4-7)+(10-13)+...+[(3n-5)-(3n-2)]
=-1-3×(n-1)/2
=(-3n+1)/2
如有其它问题请采纳此题后点求助,
n为偶数时
-1+4-7+10-.-(3n-5)+(3n-2)
=(-1+4)+(-7+10)+...+[-(3n-5)+(3n-2)]
=3×n/2
=3n/2
n为奇数时
-1+4-7+10-13+...+(3n-5)-(3n-2)
=-1+(4-7)+(10-13)+...+[(3n-5)-(3n-2)]
=-1-3×(n-1)/2
=(-3n+1)/2
如有其它问题请采纳此题后点求助,