等差数列[an}中,a1+a4+a7=39,a3+a6+a9=27,则数列前9项的和为?

题目

等差数列[an}中,a1+a4+a7=39,a3+a6+a9=27,则数列前9项的和为?

答案

a1+a4+a7=3a4=3a1+9d=39
a1+3d=13 (1)
a3+a6+a9=3a6=3a1+15d=27
a1+5d=9 (2)
(2)-(1)
d=-2
a1=19
S9=9a1+(8*9/2)d=99