设Sn是等比数列{an}的前n项和,S3,S9,S6成等差数列,且a2+a5=2am,则m=________.
在等差数列{an}中,a1=3,a4=2,则a4+a7+…+a3n+1等于________.
在等比数列{an}中,2a3-a2a4=0,则a3=________;{bn}为等差数列,且b3=a3,则数列{bn}的前5项和等于________.
在等比数列{an}中,a1=2,前n项和为Sn,若数列{an+1}也是等比数列,则Sn等于( ).
A.2n+1-2 B.3n C.2n D.3n-1
已知等差数列{an}满足2a2-+2a12=0,且{bn}是等比数列,若b7=a7,则b5b9=( )
A.2 B.4 C.8 D.16
已知正项数列{an}满足a1=1,(n+2)an+12-(n+1)+anan+1=0,则它的通项公式为( ).
A.an= B.an=
C.an= D.an=n