Sum of Fibonacci Numbers | Set 2

Namachila November 19, 2025

0 0 5 minutes read

Given a positive number N, the task is to find the sum of the first (N + 1) Fibonacci Numbers.

Examples:

Input: N = 3
Output: 4
Explanation:
The first 4 terms of the Fibonacci Series is {0, 1, 1, 2}. Therefore, the required sum = 0 + 1 + 1 + 2 = 4.

Input: N = 4
Output: 7

Naive Approach: For the simplest approach to solve the problem, refer to the previous post of this article.
Time Complexity: O(N)
Auxiliary Space: O(1)

Efficient Approach: The above approach can be optimized by the following observations and calculations:

Let S(N) represent the sum of the first N terms of Fibonacci Series. Now, in order to simply find S(N), calculate the (N + 2)^th Fibonacci number and subtract 1 from the result. The N^th term of this series can be calculated by the formula:

$F_N = \frac{(\frac{1 + \sqrt{5}}{2})^{N}}{\sqrt{5}}$

Now the value of S(N) can be calculated by (F_{N + 2} – 1).

Therefore, the idea is to calculate the value of S_N using the above formula:

Below is the implementation of the above approach:

C++

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to find the sum of
// first N + 1 fibonacci numbers
void sumFib(int N)
{
   
    // Apply the formula
    long num = (long)round(pow((sqrt(5) + 1) 
                               / 2.0, N + 2)
                           / sqrt(5));
 
    // Print the result
    cout << (num - 1);
}
 
// Driver Code
int main()
{
    int N = 3;
    sumFib(N);
    return 0;
}
 
// This code is contributed by Dharanendra L V.

Java

// Java program for the above approach
import java.io.*;
 
class GFG {
 
    // Function to find the sum of
    // first N + 1 fibonacci numbers
    public static void sumFib(int N)
    {
        // Apply the formula
        long num = (long)Math.round(
            Math.pow((Math.sqrt(5) + 1)
                         / 2.0,
                     N + 2)
            / Math.sqrt(5));
 
        // Print the result
        System.out.println(num - 1);
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        int N = 3;
        sumFib(N);
    }
}

Python3

# Python program for the above approach
import math
 
# Function to find the sum of
# first N + 1 fibonacci numbers
def sumFib(N):
   
    # Apply the formula
    num = round(pow((pow(5,1/2) + 1) \
                    / 2.0, N + 2) \
                / pow(5,1/2));
 
    # Print the result
    print(num - 1);
 
# Driver Code
if __name__ == '__main__':
    N = 3;
    sumFib(N);
 
# This code is contributed by 29AjayKumar

C#

// C# program for the above approach
using System;
class GFG
{
 
    // Function to find the sum of
    // first N + 1 fibonacci numbers
    public static void sumFib(int N)
    {
        // Apply the formula
        long num = (long)Math.Round(
            Math.Pow((Math.Sqrt(5) + 1)
                         / 2.0,
                     N + 2)
            / Math.Sqrt(5));
 
        // Print the result
        Console.WriteLine(num - 1);
    }
 
// Driver Code
static public void Main()
{
        int N = 3;
        sumFib(N);
}
}
 
// This code is contributed by jana_sayantan.

Javascript

<script>
 
// Javascript program for the above approach
 
    // Function to find the sum of
    // first N + 1 fibonacci numbers
    function sumFib(N) 
    {
        // Apply the formula
        var num =  Math.round(Math.pow((Math.sqrt(5) + 1)
        / 2.0, N + 2) / Math.sqrt(5));
 
        // Print the result
        document.write(num - 1);
    }
 
    // Driver Code
     
        var N = 3;
        sumFib(N);
 
// This code contributed by umadevi9616
 
</script>

Output:

Time Complexity: O(log n)
Auxiliary Space: O(1)

Alternate Approach: Follow the steps below to solve the problem:

The N^th Fibonacci number can also be calculated using the generating function.
The generating function for the N^th Fibonacci number is given by:

$G(X) = \frac{X}{(1 - X - X^{2})}$

Therefore, the generating function of the sum of Fibonacci numbers is given by:

$G'(X) = \frac{X}{((1 - X - X^{2}) * (1 - X))}$

After partial fraction decomposition of G'(X), the value of G'(X) is given by:

$G'(X) = \frac{a}{(a + 1) * (a - b) * (X + a)} + \frac{b}{(b + 1) * (b - a) * (X + b)} + \frac{1}{(a + 1) * (b + 1) * (X - 1)}$

where,
$a = \frac{(1 + \sqrt5)}{2}, b = \frac{(1 - \sqrt5)}{2}$

Therefore, the formula for the sum of the Fibonacci numbers is given by:

$S_{N} = \mid \frac{1}{b^{N + 1} + b^{N} + b^{N - 1} + b^{N - 2} } \mid - 1$