问题
选择题
已知Sn为等差数列{an}的前n项和,若S1=1,
|
答案
由S1=a1=1,
=4,S4 S2
得到
=4,解得d=2,4+6d 2+d
则
=S6 S4
=6a1+15d 4a1+6d
=36 16
.9 4
故选A
已知Sn为等差数列{an}的前n项和,若S1=1,
|
由S1=a1=1,
=4,S4 S2
得到
=4,解得d=2,4+6d 2+d
则
=S6 S4
=6a1+15d 4a1+6d
=36 16
.9 4
故选A