Area of decagon inscribed within the circle in C Program?

A regular decagon is a ten sided polygon with all sides and angles equal. And here we need to find the area of a decagon that is inscribed inside a circle using the radius of the circle r,

Mathematical formula for the side of the decagon inscribed in the circle,

a = r√(2-2cos36o)

(Using cosine rules)

Formula for finding area of decagon,

Area = 5*a2*(√5+2√5)/2
Area = 5 *(r√(2-2cos36))^2*(√5+2√5)/2
Area = (5r2*(3-√5)*(√5+2√5))/4

Using this formula in a program,

Example

#include <stdio.h>
#include <math.h>
int main() {
   float r = 8;
   float area = (5 * pow(r, 2) * (3 - sqrt(5))* (sqrt(5) + (2 * sqrt(5))))/ 4;
   printf("area = %f",area);
   return 0;
}

Output

area = 409.968933