等差数列{an}中,Sn是其前N项和,a1= -2008,S(2007)/2007-S(2005)/2005=2 则S(2008)=?

Question

等差数列{an}中,Sn是其前N项和,a1= -2008,S(2007)/2007-S(2005)/2005=2 则S(2008)=?

Answer 1

Sn=na1+[n(n-1)/2]dSn/n=(d/2)n+a1-d/2S(2007)/2007-S(2005)/2005=2得d=2S2008/2008=2008+(-2008)-1=-1顺便说明(2005,S2005/2005)(2007,S2007/2007)(2008,S2008/2008)在同一直线上如果题目中已知S2007、S2005 可以根...