等差数列an,a3=6,a6=11,则a9=?
问题描述:
等差数列an,a3=6,a6=11,则a9=?
答
a3=a1+2d =6
a6=a1+5d=11
3d=5 d=5/3
a1= 6-2d =8/3
a9=a1+8d=8/3 + 40/3 =16
另解
a3+a9 =2a6
a9= 2a6-a3 =22-6 =16