#include <iostream> using namespace std; what is use ?

Answer 1

If you mean to ask what is a namespace then;

https://www.w3schools.in/cplusplus-tutorial/namespace/
https://en.cppreference.com/w/cpp/language/namespace

If you mean to ask what and/or why to use `namespace std;` then you need to use the search bar, because many had asked that topic.

Answer 2

You can replace it by std::cout;
std::cin; to use cout and cin functions because they are defined in this scope of namespace std in the iostream.h file.Without including it you can not use cout ...