c++ 变量声明

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#include <iostream>
using namespace std;
//变量声明
extern int a,b;
extern int c;
extern float f;

int main()
{
  //变量定义
  int a,b;
  int c ;
  float f;

  a=10;
  b=20;
  c=a+b;
  cout << c << endl;

  f=70.0/3.0;

  cout << f<< endl;

  return 0;

 }