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

**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) f_{n}= round(f_{n-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 <bits/stdc++.h> ` `using` `namespace` `std; ` ` ` `// Approximate value of golden ratio ` `double` `PHI = 1.6180339; ` ` ` `// Fibonacci numbers upto n = 5 ` `int` `f[6] = { 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; ` `} ` |

*chevron_right*

*filter_none*

## 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. ` |

*chevron_right*

*filter_none*

## 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". ` |

*chevron_right*

*filter_none*

## 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. ` |

*chevron_right*

*filter_none*

## PHP

`<?php ` `// PHP program to find n-th ` `// Fibonacci number Approximate ` `// value of golden ratio ` `$PHI` `= 1.6180339; ` ` ` `// Fibonacci numbers ` `// upto n = 5 ` ` ` `// Function to find nth ` `// Fibonacci number ` `function` `fib (` `$n` `) ` `{ ` ` ` `global` `$PHI` `; ` ` ` `$f` `= ` `array` `(0, 1, 1, 2, 3, 5); ` ` ` ` ` `// 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` `); ` ` ` `$t` `++; ` ` ` `} ` ` ` ` ` `return` `$fn` `; ` `} ` ` ` ` ` `// Driver Code ` ` ` `$n` `= 9; ` ` ` `echo` `$n` `, ` `"th Fibonacci Number = "` `, ` ` ` `fib(` `$n` `), ` `"\n"` `; ` ` ` `// This code is contributed by aj_36 ` `?> ` |

*chevron_right*

*filter_none*

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.

## Recommended Posts:

- G-Fact 18 | Finding nth Fibonacci Number using Golden Ratio
- Deriving the expression of Fibonacci Numbers in terms of golden ratio
- Program to find last two digits of Nth Fibonacci number
- Find Index of given fibonacci number in constant time
- Check if a M-th fibonacci number divides N-th fibonacci number
- Ratio of mth and nth terms of an A. P. with given ratio of sums
- Program to find the common ratio of three numbers
- Find if it is possible to get a ratio from given ranges of costs and quantities
- Program to find the count of coins of each type from the given ratio
- Find amount to be added to achieve target ratio in a given mixture
- Number of ways to represent a number as sum of k fibonacci numbers
- Find the sum of first N odd Fibonacci numbers
- Finding number of digits in n'th Fibonacci number
- Find the GCD of N Fibonacci Numbers with given Indices
- Nth Even Fibonacci Number

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.