## How to test if a Number belongs to the Fibonacci Series in C#

To test if a number belongs to the Fibonacci series in C#, you can use the following approach:

- Check if the number is a perfect square:
- A number belongs to the Fibonacci series if and only if
`(5 * n^2 + 4)`

or`(5 * n^2 - 4)`

is a perfect square. - You can use the
`Math.Sqrt()`

function to check if a number is a perfect square by comparing its square root with the integer value of the square root.

- A number belongs to the Fibonacci series if and only if
- Implement a method to test if a number is in the Fibonacci series:
- Create a method, let’s call it
`IsFibonacci()`

, that takes an integer as input and returns a boolean indicating whether the number belongs to the Fibonacci series. - In the method, check if
`(5 * n^2 + 4)`

or`(5 * n^2 - 4)`

is a perfect square using the above approach. - Return
`true`

if either of the two conditions is met; otherwise, return`false`

.

- Create a method, let’s call it

Here’s an example implementation of the `IsFibonacci()`

method:

```
public static bool IsFibonacci(int number)
{
int term1 = 5 * number * number + 4;
int term2 = 5 * number * number - 4;
return IsPerfectSquare(term1) || IsPerfectSquare(term2);
}
private static bool IsPerfectSquare(int number)
{
int sqrt = (int)Math.Sqrt(number);
return sqrt * sqrt == number;
}
```

You can then use the `IsFibonacci()`

method to test if a given number belongs to the Fibonacci series. For example:

```
int number = 8;
bool isFibonacci = IsFibonacci(number);
Console.WriteLine($"The number {number} {(isFibonacci ? "belongs to" : "does not belong to")} the Fibonacci series.");
```

In this example, the number `8`

is tested using the `IsFibonacci()`

method, and the result is displayed in the console.

Please note that this approach works for integers within the range supported by the `int`

data type. If you need to test larger numbers, you may need to use a different approach that handles big integers or consider performance optimizations.