# C# Program - Calculate sum of Natural numbers

In Mathematics, the natural numbers are all positive numbers which is used for counting like 1, 2, 3, 4, and so on. The smallest natural number is 1.

Objective: Write a C# program which returns sum of natural numbers starting from 1 to given natural number n, (1 + 2 + 3 + ... + n).

### Method 1: Using while loop

The example below shows how to use while loop to calculate sum of first n natural numbers.

```using System;

class MyProgram {
static void Main(string[] args) {
int n = 10;
int i = 1;
int sum = 0;

//calculating sum from 1 to n
while(i <= n) {
sum += i;
i++;
}

Console.WriteLine("Sum is: " + sum);
}
}
```

The above code will give the following output:

```Sum is: 55
```

### Method 2: Using for loop

The same can be achieved using for loop. Consider the example below:

```using System;

class MyProgram {
static void Main(string[] args) {
int n = 10;
int sum = 0;

//calculating sum from 1 to n
for(int i = 1; i <= n; i++)
sum += i;

Console.WriteLine("Sum is: " + sum);
}
}
```

The above code will give the following output:

```Sum is: 55
```

### Method 3: Using Recursion

Similarly, recursion can be used to calculate the sum.

```using System;

class MyProgram {
//recursive method
static int Sum(int n) {
if(n == 1)
return 1;
else
return (n + Sum(n-1));
}

static void Main(string[] args) {
Console.WriteLine("Sum of first 10 natural numbers: " + Sum(10));
Console.WriteLine("Sum of first 20 natural numbers: " + Sum(20));
}
}
```

The above code will give the following output:

```Sum of first 10 natural numbers: 55
Sum of first 20 natural numbers: 210
```

### Method 4: Using Mathematical Formula

The sum of first n natural numbers can be mathematically expressed as:

```using System;

class MyProgram {
static void Main(string[] args) {
int n = 10;

//calculating sum from 1 to n
int sum = n*(n+1)/2;

Console.WriteLine("Sum is: " + sum);
}
}
```

The above code will give the following output:

```Sum is: 55
```

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