Why in header fule does not include .h ?

Answer 1

In C++, the style is
#include <iostream>
whereas in C, the style is
#include "stdio.h"
Your question wasn't clear so I don't know if the answer is relevant.

Answer 2

.h in c libraries is only a convention

Answer 3

When I was in high school for simple calculation like sum of two no i use this. But in this it's not working But on computer it's working
#include<iostream.h>
#include<conio.h>
void main()
{
clrscr();
int a,b,c;
cout<<"Enter first number\n";
cin>>a;
cout<<"Enter second number\n";
cin>>b;
c=a+b;
cout<<"Add="<<c;
getch();
}