diketahui barisan bilangan 1,4,8,13,..... nilai U20-U12 adalah......

Bab Barisan dan Deret
Matematika SMP Kelas VIII

1, 4, 8, 13, ... → a = 1
  3, 4, 5, .. → b = 3
    1, 1, .. → c = 1

Un = a + (n - 1) b + 1/2 x (n - 1) x (n - 2) x c

U20 = 1 + (19 x 3) + (1/2 x 19 x 18 x 1)
        = 1 + 57 + 171
        = 228

U12 = 1 + (11 x 3) + (1/2 x 11 x 10 x 1)
        = 1 + 33 + 55
        = 89

U20 - U12 = 228 - 89
                  = 139