使用的是高中数学的一个公式,p=(a+b+c)/2,s=sqrt(p*(p-a)*(p-b)*(p-c)),由正弦定理a/sinA=b/sinB=c/sinC=2r,s=0.5*a*b*sinC,所以r=a*b*c/(4*s),然后就可以求周长了。
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
1 #include <stdio.h> 2 #include <math.h> 3 #define pi 3.141592653589793 4 5 int main(void){ 6 double x1,y1,x2,y2,x3,y3,a,b,c,s,cir,r,p; 7 while(scanf("%lf %lf %lf %lf %lf %lf",&x1,&y1,&x2,&y2,&x3,&y3)!=EOF){ 8 a=sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2)); 9 b=sqrt((x1-x3)*(x1-x3)+(y1-y3)*(y1-y3)); 10 c=sqrt((x3-x2)*(x3-x2)+(y3-y2)*(y3-y2)); 11 p=(a+b+c)/2; 12 s=sqrt(p*(p-a)*(p-b)*(p-c)); 13 r=a*b*c/(4*s); 14 cir=2*pi*r; 15 printf("%.2lf ",cir); 16 } 17 return 0; 18 }