Pola bilangan 21,36,55,78,105?

36 - 21 = 15
55 - 36 = 19
78 - 55 = 23
105 - 78 = 27

Pola = n+4

Selanjutnya 136, 171

21,36,55,78,105
15 19 23. 27
4. 4. 4

a = 21
b = 15
c = 4

Un = a+(n-1)b + c(n-1)(n-2)/2
= 21+(n-1)15+4(n-1)(n-2)/2
= 21+15n-15 + 2(n²-3n+2)
= 15n+6 + 2n²-6n+4
= 2n²+9n+10