## Area of a triangle in C++ :

In this *C++* program, we will learn how to find the area of a triangle if its sides are given. We will take the sides as input from the user. This example will show you how to read user inputs in *C++* and how to do mathematical calculations.

Before moving to the program, let me quickly show you the mathematical formula to calculate the triangle area.

### Formula to calculate the area of a triangle :

We need the sides of a triangle to calculate its area. For example, if the sides of a triangle are *s1*, *s2* and *s3*, its area is *square root* of *s * (s - s1) * (s - s2) * (s - s3)*, where *s = (s1 + s2 + s3)/2*.

We will get the sides as inputs from the user and then the program will calculate the final area. We will also add two validation checks in the program :

- All sides should be positive.
- The sum of any two sides should be always greater than the third side.

These are basic validation check for a valid triangle. We will write different functions to do these validations.

Letâ€™s code :

### C++ program to find the area of a triangle :

```
#include<iostream>
#include<cmath>
using namespace std;
// 1
void isSideValid(float length){
if(length< 0){
cout<<"Error !! Please enter a valid side"<<endl;;
exit(0);
}
}
// 2
void checkAllSides(float first, float second, float third){
if(first + second <= third || first + third <= second || second + third <= first){
cout<<"Error !! Sides are not valid"<<endl;
exit(0);
}
}
int main()
{
// 3
float firstSide, secondSide, thirdSide;
float s,area;
// 4
cout<< "Enter the length of the first side : "<<endl; cin>>firstSide;
isSideValid(firstSide);
cout<< "Enter the length of the second side : "<<endl; cin>>secondSide;
isSideValid(secondSide);
cout<< "Enter the length of the third side : "<<endl; cin>>thirdSide;
isSideValid(thirdSide);
// 5
checkAllSides(firstSide,secondSide,thirdSide);
// 6
s = (firstSide + secondSide + thirdSide)/2;
// 7
area = sqrt(s * (s - firstSide) * (s - secondSide) * (s - thirdSide));
cout << "Area of the triangle is "<<area<<endl;
return 0;
}
```

### Explanation :

*The commented numbers in the above program denote the step numbers below :*

*isSideValid*function is used to check if a side entered by the user is valid or not. If it is less than*0*, it exits the program.*checkAllSides*function checks if the three sides entered by the user are valid or not. If not valid, it exits the program.- Create some floating point variables to store the sides of the triangle,
*s*and final area of the triangle. - Ask the user to enter the length of each side. Read the values and also check if each one is valid or not.
- Once all sides are read, use
*checkAllSides*function to verify if the sides are valid. - Calculate the value of
*s*. - Calculate the
*area*and print this value.

### Sample Output :

```
Enter the length of the first side :
5
Enter the length of the second side :
4
Enter the length of the third side :
3
Area of the triangle is 6
Enter the length of the first side :
10
Enter the length of the second side :
12
Enter the length of the third side :
13
Area of the triangle is 56.9951
```