Diketahui baris aritmatika 5,8,11,......,125,128,131,suku tengahnya adalah

Ut = 1/2 (5 + 131)
= 1/2 . 136
= 68

a = 5
b = 3
banyaknya suku
<=> a + (n - 1) b = S akhir
<=> 5 + (n - 1)×3 = 131
<=> (n - 1)×3 =131 - 5
<=> (n - 1)×3 = 126
<=> n - 1 = 126/3
<=> n - 1 = 42
<=> n = 42 + 1
<=> n = 43
urutan suku tengah
(n + 1) / 2 = (43 + 1)/2 = 44/2 = 22
Sn = a + (n - 1)b
S22 = 5 + (22 - 1)×3
= 5 + 21×3
= 5 + 63
= 68
jadi suku tengahnya adalah 68