> 
> 先求出3边长,由海伦公式得出面积
> S=sqrt(p*(p-a)*(p-b)*(p-c))    p=(a+b+c)/2;
> 由三角形面积公式S=1/2*a*b*sin(C)
> 和正弦定理a/sin(A)=b/sin(B)=c/sin(C)=直径
> 可得直径=a*b*c/2/S;   (a,b,c为三边长)
> 所以周长即为直径*PI
> 
> 完毕!
> 
> 下面是A过的代码!不嫌麻烦的可以看看!!!
> 
> #include<stdio.h>
> #include<math.h>
> #define PI 3.141592653589793
> double w(double a,double b,double c,double d) //求边长
> { double x;
>  x=sqrt((a-c)*(a-c)+(b-d)*(b-d));
>  return x;
> }
> double g(double o,double p,double q) //求三角形面积：海伦公式
> { double m=(o+p+q)/2;
>   m=sqrt(m*(m-o)*(m-p)*(m-q));
>   return m;
> }
> double h(double s,double u,double v,double w) //求直径
> {double y;
>  y=2*s/u/v;
>  y=w/y;
>  return y;
> }
> int main()
> {double a,b,c,d,e,f,i,j,k,l;
> while((scanf("%lf %lf %lf %lf %lf %lf",&a,&b,&c,&d,&e,&f))!=EOF){
>      i=w(a,b,c,d);
> 	 j=w(a,b,e,f);
> 	 k=w(c,d,e,f);
> 	 l=g(i,j,k);
>    l=h(l,i,j,k);
>    printf("%.2lf\n",PI*l);
> }
> return 0;
> }