Diketahui barisan bilangan 12,15,19,24,30... Nilai suku ke-12 barisan bilangan tersebut adalah

12, 15, 19, 24, 30

2a = (19 - 15) - (15 - 12)
2a = 4 - 3
2a = 1
a = 0,5

3a + b = 15 - 12
3 (0,5) + b = 3
1,5 + b = 3
b = 1,5

a + b + c = 12
0,5 + 1,5 + c = 12
2 + c = 12
c = 10

Un = an^2 + bn + c
Un = 0,5n^2 + 1,5n + 10
U12 = (0,5)(144) + (1,5)(12) + 10
U12 = 72 + 18 + 10
U12 = 100