C Exercises: Check if a given number is Fibonacci number or not

C Programming Mathematics: Exercise-26 with Solution

Write a C program to check if a given number is a Fibonacci number or not.

In mathematics, the Fibonacci numbers, commonly denoted F_n form a sequence, called the Fibonacci sequence, such that each number is the sum of the two preceding ones, starting from 0 and 1. That is, and for n > 1. By starting with 1 and 2, the first 10 terms will be: 1, 2, 3, 5, 8, 13, 21, 34, 55, 89.

Example:
Input: n = 8
Output: Is 8 a Fibonacci number? 1

Pictorial Presentation:

Sample Solution:

C Code:

#include <stdio.h>
#include <math.h>
  int isPerfectSquare(int x)
    {
       int s = (int)sqrt(x); 
       return (s*s == x);
     }  
   int isFibonacci(int x)
      {
         return isPerfectSquare(5*x*x + 4) ||
           isPerfectSquare(5*x*x - 4);
     }
    int main(void)
    {   
     //int n = 12;
	 //int n = 55;
	 int n = 8;
	  if (n>0)
		{	
		 printf("Is %d a Fibonacci number? %d",n, isFibonacci(n)); 
		} 
		return 0;        
   }

Sample Output:

Is 8 a Fibonacci number? 1

Flowchart:

C Programming Code Editor:

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