Computing Sum of n Natural numbers using different methods

sum_of_n.cpp

#include <iostream>
using namespace std;

int sum_function(int last){
  static int sum = 0;
  for(int i=1;i<=last;i++){
    sum += i;
  }
  return sum;
}

int using_while(int last){
  int i=1;
  static int sum=0;
  while(i<=last){
    sum += i;
    i++;
  }
  return sum;
}

int using_formula(int last){
  return (last)*(last+1)/2;
}

int using_recursion(int m){
  if(m == 0){
    return 0;
  }else{
    return using_recursion(m-1) + m;
  }
}

int main(){
  int n = 9;
  cout << "Using Sum funtion : " << sum_function(n) << endl;
  cout << "Using while loop : " << using_while(n) << endl;
  cout << "Using Formula : " << using_formula(n) << endl;
  cout << "Using Recursion : " << using_recursion(n) << endl;
  return 0;
}