n n 9 + 3 - 12 ------------ n 3 - 1 - 1
n 2*n -12 + 3 + 3 --------------- n -2 + 3
n n -12 + 3 + 9 ------------- n -2 + 3
(-12.0 + 3.0^n + 9.0^n)/(-2.0 + 3.0^n)
n -10 + 9 1 + -------- n -2 + 3