等差数列{an}中,a1+a4+a7=36,a2+a5+a8=33,则a3+a6+a9=_.
问题描述:
等差数列{an}中,a1+a4+a7=36,a2+a5+a8=33,则a3+a6+a9=______.
答
由等差数列的性质可得,a1+a4+a7=3a4=36,a2+a5+a8=3a5=33
∴a4=12,a5=11,d=-1
a3+a6+a9=3a6=3(a5-1)=30
故答案为:30