# Mathematical Constants in C++

In this tutorial, we will learn how to use the mathematical constants in C++.

We use mathematical constants in various places from simple mathematical calculations to physics to chemistry and even financial problems. C++ has a few predefined constants in its math library which we can use to access the values wherever needed in our program.

We use the following header file :

```#define _USE_MATH_DEFINES
#include <cmath>```

Here, _USE_MATH_DEFINES is a #define macro. C++ has the following constants with their values :

SymbolExpressionValue
M_Ee2.71828182845904523536
M_LOG2Elog2(e)1.44269504088896340736
M_LOG10Elog10(e)0.434294481903251827651
M_LN2ln(2)0.693147180559945309417
M_LN10ln(10)2.30258509299404568402
M_PIpi3.14159265358979323846
M_PI_2pi/21.57079632679489661923
M_PI_4pi/40.785398163397448309616
M_1_PI1/pi0.318309886183790671538
M_2_PI2/pi0.636619772367581343076
M_2_SQRTPI2/sqrt(pi)1.12837916709551257390
M_SQRT2sqrt(2)1.41421356237309504880
M_SQRT1_21/sqrt(2)0.707106781186547524401

Here I have written a simple program to access the values :

## Program to show Mathematical Constants in C++ :

```#define _USE_MATH_DEFINES
#include<cmath>
#include<iostream>
using namespace std;
int main()
{
cout << "log10(e) = " << M_LOG10E<< endl << "exp = " << M_E << endl << "sqrt(2) = " << M_SQRT2 << endl;
return 0;
}```

Output :

```log10(e) = 0.434294
exp = 2.71828
sqrt(2) = 1.41421```

Now, we write another program to find the area of a circle using M_PI constant.

## Program to find the area using M_PI :

```#define _USE_MATH_DEFINES
#include<cmath>
#include<iostream>
using namespace std;
int main()
{
int r;
float a;
cout << "Enter the value of radius : ";
cin >> r;
a = M_PI * r * r ;
cout << "area of the circle is : " << a << endl;
return 0;
}```

Output :

```Enter the value of radius : 9
area of the circle is : 254.469```

Hope this was helpful. Enjoy Coding!