Question

求这道题过程

Answer 1

该数列为等差数列，设公差为d，所以a8-a3=5d=9-5=4
d等于五分之四
a11＝a8+3d＝9+3*4/5＝五分之五十七

Answer 2

依题知{an}为等差数列，则有an = a1+ (n-1)d (d为公差)
a3 = a1 + 2d = 5, a8 = a1 + 7d = 9， 后式减前式有5d=4, d=4/5, a1=17/5
a11= a1 + 10d = 17/5 + 8 = 11又2/5

追问

问一下a1等于五分之十七怎么来的

追答

因为d=4/5, 代入a3 = a1 + 2d = 5或a8 = a1 + 7d = 9中可以算出a1.

追问

嗯，谢谢！