rumus suku ke n pada barisan bilangan bertingkat 5,9,15,23....adalah....

Quadratic Sequence.

5, 9, 15, 23, ...
a = 5
b = 4
c = 2
The n-th term formula is:
Un = a / 0! + b(n - 1) / 1! + c(n - 1)(n - 2) / 2!
      = 5 / 0! + 4(n - 1) / 1! + 2(n - 1)(n - 2) / 2!
      = n² + n + 3