问题
填空题
已知数列{an}为等差数列,若a3+a4+a5=9,则S7=______.
答案
∵数列{an}为等差数列,且a3+a4+a5=9,
∴a4=3,
∴S7=
(a1+a7)=7a4=21.7 2
故答案为:21.
已知数列{an}为等差数列,若a3+a4+a5=9,则S7=______.
∵数列{an}为等差数列,且a3+a4+a5=9,
∴a4=3,
∴S7=
(a1+a7)=7a4=21.7 2
故答案为:21.