两个等差数列{an}和{bn},其前n项和分别为Sn,Tn,且SnTn=7n+2n+3,则a2+a20b7+b15等于( ) A.94 B.378 C.7914 D.14924
问题描述:
两个等差数列{an}和{bn},其前n项和分别为Sn,Tn,且
=Sn Tn
,则7n+2 n+3
等于( )
a2+a20
b7+b15
A.
9 4
B.
37 8
C.
79 14
D.
149 24
答
因为:
=
a2+a20
b7+b15
a1+a21
b1+b21
=
(a1+a21)21 2
(b1+b21) 21 2
=
=S21 T21
=7×21+2 21+3
.149 24
故选:D.