# Find nth Fibonacci number using Golden ratio

Fibonacci series = 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, ……..

Different methods to find nth Fibonacci number are already discussed. Another simple way of finding nth Fibonacci number is using golden ratio as Fibonacci numbers maintain approximate golden ratio till infinite.
Golden ratio: Examples:

Input : n = 9
Output : 34

Input : n = 7
Output : 13


## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Approach:
Golden ratio may give us incorrect answer.
We can get correct result if we round up the result at each point.

nth fibonacci number = round(n-1th Fibonacci number X golden ratio)
fn = round(fn-1 * )


Till 4th term, the ratio is not much close to golden ratio (as 3/2 = 1.5, 2/1 = 2, …). So, we will consider from 5th term to get next fibonacci number. To find out the 9th fibonacci number f9 (n = 9) :

     f6 = round(f5 * ) = 8
f7 = round(f6 * ) = 13
f8 = round(f7 * ) = 21
f9 = round(f8 * ) = 34

Note: This method can calculate first 34 fibonacci numbers correctly. After that there may be difference from the correct value.

Below is the implementation of above approach:

## CPP

 // CPP program to find n-th Fibonacci number  #include  using namespace std;     // Approximate value of golden ratio  double PHI = 1.6180339;     // Fibonacci numbers upto n = 5  int f = { 0, 1, 1, 2, 3, 5 };     // Function to find nth  // Fibonacci number  int fib (int n)  {      // Fibonacci numbers for n < 6      if (n < 6)          return f[n];         // Else start counting from       // 5th term      int t = 5, fn = 5;         while (t < n) {           fn = round(fn * PHI);           t++;      }         return fn;     }     // driver code  int main()  {      int n = 9;         cout << n << "th Fibonacci Number = "           << fib(n) << endl;         return 0;  }

## Java

 // Java program to find n-th Fibonacci number     class GFG  {      // Approximate value of golden ratio      static double PHI = 1.6180339;             // Fibonacci numbers upto n = 5      static int f[] = { 0, 1, 1, 2, 3, 5 };             // Function to find nth      // Fibonacci number      static int fib (int n)      {          // Fibonacci numbers for n < 6          if (n < 6)              return f[n];                 // Else start counting from           // 5th term          int t = 5;          int fn = 5;                 while (t < n) {              fn = (int)Math.round(fn * PHI);              t++;          }                 return fn;       }             // Driver code      public static void main (String[] args)       {          int n = 9;          System.out.println(n + "th Fibonacci Number = "                                                 +fib(n));      }  }     // This code is contributed by Anant Agarwal.

## Python3

 # Python3 code to find n-th Fibonacci number     # Approximate value of golden ratio  PHI = 1.6180339    # Fibonacci numbers upto n = 5  f = [ 0, 1, 1, 2, 3, 5 ]     # Function to find nth  # Fibonacci number  def fib ( n ):         # Fibonacci numbers for n < 6      if n < 6:          return f[n]         # Else start counting from      # 5th term      t = 5     fn = 5            while t < n:          fn = round(fn * PHI)          t+=1            return fn     # driver code  n = 9 print(n, "th Fibonacci Number =", fib(n))     # This code is contributed by "Sharad_Bhardwaj".

## C#

 // C# program to find n-th Fibonacci  // number  using System;     class GFG {             // Approximate value of golden ratio      static double PHI = 1.6180339;             // Fibonacci numbers upto n = 5      static int []f = { 0, 1, 1, 2, 3, 5 };             // Function to find nth      // Fibonacci number      static int fib (int n)      {                     // Fibonacci numbers for n < 6          if (n < 6)              return f[n];                 // Else start counting from           // 5th term          int t = 5;          int fn = 5;                 while (t < n) {              fn = (int)Math.Round(fn * PHI);              t++;          }                 return fn;       }             // Driver code      public static void Main ()       {          int n = 9;                     Console.WriteLine(n + "th Fibonacci"                     + " Number = " + fib(n));      }  }     // This code is contributed by vt_m.

## PHP

 

Output:

9th Fibonacci Number = 34


We can optimize above solution work in O(Log n) by using efficient method to compute power.

The above method may not always produce correct results as floating point computations are involved. This is the reason, this method is not used practically even if it can be optimized to work in O(Log n). Please refer below MIT video for more details.

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