1) 1, 9, 16, 22, ... Un : ? 2) 4,9,16,25,... Un : ?

Bab Barisan dan Deret
Matematika SMP Kelas IX

1] 1, 9, 16, 22, ... → a = 1
       8,  7, 6, ... → b = 8
           1, 1,  → c = 1

Un = a + (n - 1) x b + 1/2 x (n - 1) x (n - 2) x c
Un = 1 + (n - 1) x 8 + 1/2 x (n² - 3n + 2) x 1
Un = 1 + 8n - 8 + 1/2 n² - 3/2 n + 1
Un = 1/2 n² + 13/2 n - 6
Un = 1/2 x (n² + 13n - 12)

2] 4, 9, 16, 25, ...
U1 = 4 = (1 + 1)²
U2 = 9 = (2 + 1)²
U3 = 16 = (3 + 1)²
U4 = 25 = (4 + 1)²
Un = (n + 1)²