Answer:
f(n) = f(n-1) + 8ⁿ⁻¹
Explanation:
The series is 5, 13, 29, 61, 125
13 - 5 = 8
29 - 13 = 16
61 - 29 = 32
125 - 61 = 64
Here, f(1) = 5
∴ f(2) = f(1) + 8¹ = 5 + 8 = 13
f(3) = f(2) + 8² = 13 + 16 = 29
f(4) = f(3) + 8³ = 29 + 32 = 61
And so on
∴ f(n) = f(n-1) + 8ⁿ⁻¹