若{an}是个等差数列,前n项和为sn,已知S4=6 S8=20 求S12
问题描述:
若{an}是个等差数列,前n项和为sn,已知S4=6 S8=20 求S12
答
因为Sn=na1+(n-1)d
S4=4a1+3d
S8=8a1+7d
解得a1= - 4.5,d=8
S12= - 4.5*12+11*8
=34