minta tolong penjelasanya

Bab Barisan dan Deret
Matematika SMP Kelas VIII

1, 3, 6, ....  → a = 1
  2, 3, ... → b = 2
    1, ... → c = 1

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

U8 = 1 + (8 - 1) . 2 + (1/2) . (8 - 1) . (8 - 2) . 1
U8 = 1 + 7 . 2 + (1/2) . 7 . 6 
U8 = 1 + 14 + 21
U8 = 36