不知道你想问的是不是这个意思...
#include"stdio.h"
void main()
{
   int a,b,c,i;
  scanf("%d%d%d",&a,&b,&c);
  i=a+b;
 if(c==i)
  printf("第三个数是前两个数的和");
else
 printf("第三个数不是前两个数的和");

 }

#include<iostream.h>
void main()
{int a,b,c;
    void pd(int a,int b,int c );
    cin>>a>>b>>c;
    pd(a,b,c);
}
void pd(int a,int b,int c)
{
if(a+b==c)
cout<<"deng yu ";
else
cout<<"bu deng yu ";
}

#include"stdio.h"
#define P printf
#define S scanf
void main()
{int kio, ha,ge,ms;
P("please input two integers:");
s("%d%d\n",&kio,&ha);
ge=kio+ha;
p("Input another integer:");
S("%d\n",&ms);
if(ms==ge)
P("the third interger is the number1");
else
p("it isn't the number! ");
getch();
return 0;
}

#include"iostream.h"
void main()
{int se,de,fe,he;
he=se+de;
void pt(int he,int fe);
cin>>se>>de>>fe;
pt(he,fe);
}
void pt (int he,int fe)
{if(he==fe)
cout<<"deng yu";
else
cout<<"bu deng yu";
}

// sdf.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"


int main(int argc, char* argv[])
{

       int a,b,c,i;
       int flag;
       do
       {
         scanf("%d %d %d",&a,&b,&c);
         i=a+b;
         if(c==i)
          printf("第三个数是前两个数的和!\n");
         else
          printf("第三个数不是前两个数的和\n");
         printf("是否继续：1：是  0：否");
         printf("\n");
         scanf("%d",&flag);
       }while(flag);

        printf("\n");
        printf("Hello World!\n");
        return 0;
}

5楼的一点都不完美，程序一直执行退不出去。得用一个选择开关语句。希望楼主认可。