OK ! Thanks.
I think what you have calculated a simplified closed form of value
given by the recurrence relation.
[given recurrence relation F(n) is not for time complexity]
example : $F_n=\frac{1}{\sqrt{5}}\left(\frac{1+\sqrt{5}}{2}\right)^n- \frac{1}{\sqrt{5}}\left(\frac{1-\sqrt{5}}{2}\right)^n$ for the fibonacci recurrence relation which depends on two previous terms. Here although F(n) grows exponentially, we know that if we use memoization or bottom up dp we can achive a linear time complexity to calculate F(n).
Additionaly : If we use matrix exponentiation like below,
$\left[ \begin{matrix} 1 && 1 \\ 1 && 0 \end{matrix} \right]^n = \left[ \begin{matrix} F(n+1) && F(n) \\ F(n) && F(n-1) \end{matrix} \right]$
Then T(n) for F(n) becomes $O(\log n)$
So my point is, What is the best time complexity we can achive to solve such a generic recurrence relation depending on k previous terms ? What if such questions asked in exam without specifying any algorithm ?