数列{an}的前n项和Sn = n2+2n+5,则a6+a7+a8 =
问题描述:
数列{an}的前n项和Sn = n2+2n+5,则a6+a7+a8 =
答
a6+a7+a8
=S8-S5
=8²+2×8+5-(5²+2×5+5)
=45