题目内容
已知{an}是等差数列,且a2+a3+a8+a9=24,则a5+a6=
A.12
B.16
C.20
D.24
设Sn是等差数{an}的前n项和,已知S6=36,Sn=324,若Sn-6=144(n>6),则n等于
A.15 B.16 C.17 D.18