1.若{an}是等差数列,且a1+a4+a7=45,a2+a5+a8=39,则a3+a6+a9=( )
A.39 B.20
C.19.5 D.33
2.等差数列{an}中,a1+a5=10,a4=7,则数列{an}的公差为( )
A.1 B.2
C.3 D.4
3.已知等差数列{an}满足a1+a2+a3+…+a101=0,则有( )
A.a1+a101>0 B.a2+a101<0
C.a3+a99=0 D.a51=51
4.已知{an}、{bn}是两个等差数列,其中a1=3,b1=-3,且a20-b20=6,那么a10-b10的值为( )
A.-6 B.6
C.0 D.10
二、填空题
5.等差数列{an}中,a1+3a8+a15=120,则2a9-a10的值是________.
6.若5,x,y,z,21成等差数列,则x+y+z=________.
7.17+3,13-3的等差中项为________.