# Formula for fibonacci(a+b).

Is there any general formula for fibonacci(A+B)?

I have tried to derive it , and got following results. \begin{align} &fib(a+1)=1*fib(a)+fib(a-1)\\ &fib(a+2)=2*fib(a)+fib(a-1)\\ &fib(a+3)=fib(a+2)+fib(a+1)=\!\text{(sum of above two formulas)}\!=3fib(a)+2fib(a-1)\\ &fib(a+4)=5fib(a)+3fib(a-1)\\ &\qquad\vdots\\ &fib(a+b)=fib(b+1)*fib(a)+fib(b)*fib(a-1) \end{align}

Is this formula correct or there is something wrong..?

• This is correct! You may want to peruse Wikipedia's Fibonacci page, because they have this and more: en.wikipedia.org/wiki/Fibonacci_number – Matt Groff Apr 6 '16 at 15:58
• this appears to be related to an active competition, codechef.com/APRIL16/problems/FIBQ . I note that one of your prior questions was locked because it, too, was from a codechef competition (math.stackexchange.com/questions/1686737/…). – lulu Apr 6 '16 at 16:09
• @lulu .! I am learning about properties of fibonacci numbers. Main motive of long contest is learning , you can learn lot of things . I didnot ask direct answer for that question . I just wanted to know that whatever i am doing is correct or not ...! – Winchester Allpesh Apr 6 '16 at 16:55
• Your prior question was exactly equivalent to a (then active) codechef competition problem. In neither case did you cite the contest as the source for your question. Most people here would, I think, want to hold off on answering until the competition was done. At a minimum, you should link to the ongoing contest so people can decide for themselves if they want to help. – lulu Apr 6 '16 at 17:04

This follows from the matrix formulation, which is well worth knowing and easily proved: $$\begin{pmatrix}1&1\\1&0\end{pmatrix}^n= \begin{pmatrix}F_{n+1}&F_n\\F_n&F_{n-1}\end{pmatrix}$$
Let $A=\begin{pmatrix}1&1\\1&0\end{pmatrix}$. Then $A^{a+b}=A^{a}A^b$. Just read the corresponding entries.