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

1,4,8,13...
↓
3,4,5,...
↓
1,1,1...

Un = a/0! + b(n-1)/1! + c(n-1)(n-2)/2!
Un = 1 + 3(n-1) + ½(n²-3n+2)
Un = 3n -2 + ½(n²-3n+2)
Un = ½(6n-4 + n²-3n+2)
Un = ½(n² +3n -2)

U20 - U12 = ½(20²+3.20-2) - ½(12²+3.12-2)
U20 - U12 = ½(458) - ½(178)
U20 - U12 = 229 - 89
U20 - U12 = 140