# C++ Program for How to check if a given number is Fibonacci number?

• Difficulty Level : Basic
• Last Updated : 14 Jan, 2022

Given a number ‘n’, how to check if n is a Fibonacci number. First few Fibonacci numbers are 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 141, ..
Examples :

```Input : 8
Output : Yes

Input : 34
Output : Yes

Input : 41
Output : No```

Following is an interesting property about Fibonacci numbers that can also be used to check if a given number is Fibonacci or not.
A number is Fibonacci if and only if one or both of (5*n2 + 4) or (5*n2 – 4) is a perfect square (Source: Wiki).

## CPP

 `// C++ program to check if x is a perfect square``#include ``#include ``using` `namespace` `std;` `// A utility function that returns true if x is perfect square``bool` `isPerfectSquare(``int` `x)``{``    ``int` `s = ``sqrt``(x);``    ``return` `(s * s == x);``}` `// Returns true if n is a Fibonacci Number, else false``bool` `isFibonacci(``int` `n)``{``    ``// n is Fibonacci if one of 5*n*n + 4 or 5*n*n - 4 or both``    ``// is a perfect square``    ``return` `isPerfectSquare(5 * n * n + 4) || isPerfectSquare(5 * n * n - 4);``}` `// A utility function to test above functions``int` `main()``{``    ``for` `(``int` `i = 1; i <= 10; i++)``        ``isFibonacci(i) ? cout << i << " is a Fibonacci Number \n"``                       ``: cout << i << " is a not Fibonacci Number \n";``    ``return` `0;``}`
Output:
```1 is a Fibonacci Number
2 is a Fibonacci Number
3 is a Fibonacci Number
4 is a not Fibonacci Number
5 is a Fibonacci Number
6 is a not Fibonacci Number
7 is a not Fibonacci Number
8 is a Fibonacci Number
9 is a not Fibonacci Number
10 is a not Fibonacci Number```

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