求这道题的详细解析 基础不好 谢谢

Question

如题所述

Answer 1

an=a1+(n-1)d
a10=30
a1+9d=30 (1)
a20=50
a1+19d=50 (2)
(2)-(1)
10d=20
d=2
from (1)
a1+18 =30
a1= 12
an = 12 + 2(n-1) =2n +10
----------
Sn =242
( 2n+10 +12 )n/2=242
(n+11)n=242
n^2+11n-242=0
(n-11)(n+22)=0
n=11

Answer 2

参考

Answer 3

图