Suatu barisan arimatika memiliki U9 = 23 dan U13 = 31. Jumlah 25 suku pertam barisan tersebut adalah

Barder

b = U13- U9 / 13- 9
= 31- 23/ 4
= 2

Maka
U9 = 23
a + 8b = 23
a + 16= 23
a = 7

S25
= 25/2 [ 2(7) + 24(2)]
= 25 (7+ 24)
= 25(31)
= 775

U13 = a + 12b= 31
U9 = a+8b = 23
______________-
4 b = 8
b=2

a+8b=23
a+8(2)=23
a+16=23
a=7

S25= 25/2×(2(7)+24(2)
25/2×(14+48)
=25/2×(62)
=155