Rumus suku ke-n dari barisan –1, 4, 11, 20, ... adalah ....

a = -1
b1 = 4-(-1) = 5
b2 = 11-4 = 7
b3 = 20-11 = 9
s1 = s2 = 7-5 = 9-7 = 2

2A = s1
A = 2/2 = 1
3A + B = b1
3x1 + B = 5
B = 5-3 = 2

A+B+C = a
1 + 2 + C = -1
C = -1-3 = -4

Un = An² + Bn + C
Un = n² + 2n - 4