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Check if a number is Fibonacci number


Fibonacci numbers are numbers such that each number is the result of adding two preceding numbers. A sequence of such numbers is called as fibonacci numbers.

Fibonacci sequence start with 1, 1 as the first 2 terms.
Fibonacci sequence is 1 , 1 , 2 , 3 , 5 , 8 , 13 , 21 , 34 , ....

If we want to check if a number is a fibonacci number or not we only need to check if 
5*number*number - 4  or 5*number*number + 4 is a perfect square or not.

To see the derivation of the above formula click here.

C++ Program

Sample input and output to check the program


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