已知等差数列{an}满足a3=7,a5+a7=26{an}的前n项和为Sn求a4及Sn
1个回答
a5+a7=a3+2d+a3+4d=26
d=2
a4=a3+d=7+2=9
Sn=a1n+(n-1)nd2
=(a3-2d)n+n(n-1)
=n(n+2)
相关问题
已知等差数列{an}满足:a3=7,a5+a7=26,{an}的前n项和为Sn.求an及Sn.
已知等差数列{an}满足:a3=7,a5+a7=26.{an}的前n项和为Sn.(1)求an及Sn; (2)令bn=1/
已知等差数列(an)满足:a3=7,a5+a7=26,(an)的前n项和为Sn
已知等差数列{an}满足:a3=7,a5+a7 =26,an的前n项和为sn.
己知等差数列(an)满足:a3=7,a5+a7=26,(an)前n项和为S(1)求an及Sn(2)令bn=1/Sn(n属
(2014•洛阳二模)已知等差数列{an}满足:a3=4,a5+a7=14,{an}的前n项和为Sn.
已知等差数列{an}中,满足a3*a7=-16,a4+a6=0,求数列{an}的前n项和Sn
已知an是等差数列,a3=5,a6=11 sn是前n项和.求an通项公式及前n项和Sn
等差数列an的前n项和为Sn,已知a5=11 a8=5求an和Sn
已知等差数列{an}中,a3a7=-16,a4+a6=0,求{an}的前n项和Sn