Language C π methods

#include <stdio.h>

#include <math.h>

int main ()

{ 

int r; 

double PI,s; 

scanf("%d",&r); 

PI = atan (1.0) * 4; 

s = PI * r * r; 

printf("%.7f",s);  

return 0;

}

Here atan arctangent function arctan, tangent function tan (π / 4) = 1, so atan (1) = π / 4, pi = 4.0 * atan (1.0) = 3.14159 ......

Reference article from: https: //blog.csdn.net/u013866359/article/details/41287399