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The Circumference of the Circle
438.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#define PI 3.141592653589793
int main(int argc, char *argv[])
{
char buf[1024];
double a1,a2,b1,b2,c1,c2;
double circ;
while(fgets(buf, 1024, stdin)) {
double p1,p2,pm,pn,q1,q2,qm,qn; /* y = pm*x + pn */
double x,y;
if (sscanf(buf, "%lf %lf %lf %lf %lf %lf", &a1, &a2, &b1, &b2, &c1, &c2)!=6) {
exit(1);
}
if (a2==b2) {
double t;
t=a1; a1=c1; c1=t;
t=a2; a2=c2; c2=t;
} else if (a2==c2) {
double t;
t=a1; a1=b1; b1=t;
t=a2; a2=b2; b2=t;
}
p1=(a1+b1)/2;
p2=(a2+b2)/2;
pm=(b1-a1)/(a2-b2);
pn=p2-pm*p1;
q1=(a1+c1)/2;
q2=(a2+c2)/2;
qm=(c1-a1)/(a2-c2);
qn=q2-qm*q1;
x = (qn-pn)/(pm-qm);
y = pm*x + pn;
circ = 2*PI*(sqrt((x-a1)*(x-a1)+(y-a2)*(y-a2)));
printf("%.2f\n", circ);
}
exit(0);
}