What is the mechanism behind the fibnocci codes?

Answer 1

its just like factorial but the difference is that you are adding the next number instead of multiplying them as in factorial .

Answer 2

the first and second number of Fibonacci series are 1. and the next numbers are sum of previous two numbers
So, a[n] = a[n-1] + a[n-2]

Answer 3

main concept is n + (n - 1).