# Find the previous fibonacci number

Given aFibonacci number N , the task is to find the previous Fibonacci number.

#### Examples:

Input:N = 8

Output:5

5 is the previous fibonacci number before 8.

Input:N = 5

Output:3

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

Approach: The ratio of two adjacent numbers in the Fibonacci series rapidly approaches ((1 + sqrt(5)) / 2) . So if N is divided by ((1 + sqrt(5)) / 2) and then rounded, the resultant number will be the previous Fibonacci number.

Below is the implementation of the above approach:

## C++

filter_none

edit

close

play_arrow

brightness_4

code

`// C++ implementation of the approach`
`#include <bits/stdc++.h>`
` `
`using` `namespace` `std;`
` `
`// Function to return the previous`
`// fibonacci number`
`int` `previousFibonacci(` `int` `n)`
`{`
`  ` `double` `a = n / ((1 +` `sqrt` `(5)) / 2.0);`
`  ` `return` `round(a);`
`}`
` `
`// Driver code`
`int` `main()`
`{`
`  ` `int` `n = 8;`
`  ` `cout << (previousFibonacci(n));`
`}`
` `
`// This code is contributed by Mohit Kumar`

chevron_right

filter_none

## Python3

filter_none

edit

close

play_arrow

brightness_4

code

`# Python3 implementation of the approach `
`from` `math` `import` `*`
` `
`# Function to return the previous `
`# fibonacci number `
`def` `previousFibonacci(n): `
`  ` `a` `=` `n` `/` `((` `1` `+` `sqrt(` `5` `))` `/` `2.0` `)`
`  ` `return` `round` `(a) `
` `
`# Driver code `
`n` `=` `8`
`print` `(previousFibonacci(n)) `

chevron_right

filter_none

## C#

filter_none

edit

close

play_arrow

brightness_4

code

`// C# implementation of the approach`
`using` `System;`
` `
`class` `GFG`
`{`
`   `
`// Function to return the previous`
`// fibonacci number`
`static` `int` `previousFibonacci(` `int` `n)`
`{`
`  ` `double` `a = n / ((1 + Math.Sqrt(5)) / 2.0);`
`  ` `return` `(` `int` `)Math.Round(a);`
`}`
` `
`// Driver code`
`public` `static` `void` `Main()`
`{`
`  ` `int` `n = 8;`
`  ` `Console.Write(previousFibonacci(n));`
`}`
`}`
` `
`// This code is contributed by Akanksha_Rai`

chevron_right

filter_none

Output:

My Personal Notes arrow_drop_up Check out this Author'scontributed articles.

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.

Improved By : mohit kumar 29