In C++, variables are used to store and manipulate data. They are like containers that can hold different types of values, such as numbers or strings.

To declare a variable in C++, you need to specify its data type and name. For example, to declare an integer variable x and assign it the value 5, you would write:

1int x = 5;

You can then use this variable in calculations or to perform other operations. For instance, you can declare another integer variable y with the value 3, and calculate the sum of x and y using the addition operator +:

1int sum = x + y;

Constants, on the other hand, are like variables but with values that cannot be changed once they are assigned. They are useful when you have a value that remains constant throughout your program. To declare a constant in C++, you use the const keyword. For example, to declare a constant named PI with the value 3.14159, you would write:

1const int PI = 3.14159;

You can then use this constant in calculations or other operations, just like a variable. For instance, you can declare a double variable radius and assign it the value 2.5, and calculate the area of a circle using the formula PI * radius * radius.

Here's an example that demonstrates declaring and using variables and constants in C++ for mathematical calculations:

1#include <iostream>
2using namespace std;
3
4int main() {
5  // Declaring and using variables in C++
6  int x = 5;
7  int y = 3;
8  int sum = x + y;
9
10  // Declaring and using constants in C++
11  const int PI = 3.14159;
12  double radius = 2.5;
13  double area = PI * radius * radius;
14
15  // Outputting the results
16  cout << "Sum: " << sum << endl;
17  cout << "Area: " << area << endl;
18
19  return 0;
20}

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#include <iostream>

using namespace std;

​

int main() {

  // Declaring and using variables in C++

  int x = 5;

  int y = 3;

  int sum = x + y;

​

  // Declaring and using constants in C++

  const int PI = 3.14159;

  double radius = 2.5;

  double area = PI * radius * radius;

​

  // Outputting the results

  cout << "Sum: " << sum << endl;

  cout << "Area: " << area << endl;

​

  return 0;

}