C program to find the area of ​​a decagon inscribed in a circle?

Here we will learn how to get the area of ​​a decagon inside a circle. The radius is given. The side of the decagon is "a".

As we all know, the side lengths of a decagon are as follows-

Example

#include <iostream>
#include <cmath>
using namespace std;
float area(float r) {
   if (r < 0) //if r is negative it is invalid
      return -1;
   float area = (5 * pow(r, 2) * (3 - sqrt(5)) * (sqrt(5) + (2 * sqrt(5)))) / 4;
   return area;
}
int main() {
   float r = 8;
   cout << "Area : " << area(r);
}

Output

Area : 409.969

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