等差数列{An}中,Sn是其前n项和,A1=-2011,S2007/2007-S2005/2005=2,求S2011
问题描述:
等差数列{An}中,Sn是其前n项和,A1=-2011,S2007/2007-S2005/2005=2,求S2011
答
设公差为d
An=-2011+( n - 1)d
Sn/n=(-2011+(n -1)d-2011)/2
S2007/2007-S2005/2005
S2007/2007=-4022+2006d S2005/2005=-4022+2004d
S2007/2007-S2005/2005=2d=2
d=1
Sn=(-4023+n)n/2
S2011=-2023066