this has been demonstrated by a simple program.......

in C we ourselves can write a function for addition and
include wherver we want to add numbers.....

1) first write the function definition for the function u
are gonna write for addition....in a new file

int addition_arthmetic(int a, int b)
{
return(a+b);
}
2) dont run or compile this , directly save this file in .c
extension.. let us say "add.c".

3) then open a new file and write the program for addition
by getting the 2 i/p from the user...

#inclue<stdio.h>
#include<conio.h>
#include "add.c"  // we are including tthe file add.c"

void main()
{
int a,b,c;
printf("enter the values for a&b");
scanf("%d%d",&a,&b);
c=addition_arthmetic(a,b);// we call the function defined in
add.c
printf("\n\n the added value of a&b is :%d",c);
getch();
}

thank u

#include<stdio.h>
#include<conio.h>
void main()
{
int a,b,i;
int sum=0;
clrscr();
printf("Enter Two Nos");
scanf("%d%d",&a,&b);
for(i=0;i<a;i++)
sum=sum+1;
for(i=0;i<b;i++)
sum=sum+1;
printf("Sum:%d",sum);
getch();
}

#include<stdio.h>
#include<conio.h>
void main()
{
 int add(int,int);
 int a,b;
 clrscr();
 printf("Enter two numbers: ");
 scanf("%d %d",&a,&b);
 printf("The Sum is: %d",add(a,b));
 getch();
}
int add(int x,int y)
{
 return(x+y);
}