suku ke-n pada barisan 5,9,15,23,.... adalah

5......9........15......23,...
...+4.....+6......+8
........+2.....+2

2a = 2
a = 2/2
a = 1

3a + b = 4
3(1) + b = 4
3 + b = 4
b = 4 - 3
b = 1

a + b + c = 5
1 + 1 + c = 5
2 + c = 5
c = 5 - 2
c = 3

Un = an^2 + bn + c
Un = 1n^2 + 1n + 3
Un = n^2 + n + 3