HDU4720+三角形外接圆

 /*
 几何
 求给定三角形的外接圆圆心
 方法：求解二元方程组
 */
 #include<stdio.h>
 #include<string.h>
 #include<math.h>
 #include<stdlib.h>
 const double pi = acos(-1.0);
 const double eps = 1e-;
 struct Point{
     double x,y;
 };
 struct Circle{
     Point center;
     double r;
 };
 Point a,b,c,tp;
 Circle cc;

 double dis( Point a,Point b ){
     return sqrt( (a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y) );
 }

 bool JudgeTriangle( Point a,Point b,Point c ){
     double len1 = dis( a,b );
     len1 *= len1;
     double len2 = dis( a,c );
     len2 *= len2;
     double len3 = dis( b,c );
     len3 *= len3;
     if( len1+len2-len3<=||len1+len3-len2<=||len2+len3-len1<= ) return false;
     else return true;
 }/*判断是否是钝角三角形*/

 void GetCircle( Point a,Point b,Point c ){
     bool judge = JudgeTriangle( a,b,c );
     if( judge==false ){
         double ans;
         int op ;
         double len1 = dis( a,b );
         ans = len1;
         op = ;
         double len2 = dis( a,c );
         if( len2>ans ){
             ans = len2;
             op = ;
         }
         double len3 = dis( b,c );
         if( len3>ans ){
             ans = len3;
             op = ;
         }
         cc.r = ans / 2.0;
         if( op== ){
             cc.center.x = ( a.x+b.x )/2.0;
             cc.center.y = ( a.y+b.y )/2.0;
         }
         else if( op== ){
             cc.center.x = ( a.x+c.x )/2.0;
             cc.center.y = ( a.y+c.y )/2.0;
         }
         else {
             cc.center.x = ( b.x+c.x )/2.0;
             cc.center.y = ( b.y+c.y )/2.0;
         }
         return ;
     }/*钝角三角形*/
     double x1 = a.x,x2 = b.x,x3 = c.x;
     double y1 = a.y,y2 = b.y,y3 = c.y;
     cc.center.x=((y2-y1)*(y3*y3-y1*y1+x3*x3-x1*x1)-(y3-y1)*(y2*y2-y1*y1+x2*x2-x1*x1))/(*(x3-x1)*(y2-y1)-*((x2-x1)*(y3-y1)));
     cc.center.y=((x2-x1)*(x3*x3-x1*x1+y3*y3-y1*y1)-(x3-x1)*(x2*x2-x1*x1+y2*y2-y1*y1))/(*(y3-y1)*(x2-x1)-*((y2-y1)*(x3-x1)));
     cc.r = dis( cc.center,a );
     return ;
 }/*求外接圆圆心*/ 

 int main(){
     int T;
     scanf("%d",&T);
     int ca = ;
     while( T-- ){
         scanf("%lf%lf%lf%lf%lf%lf%lf%lf",&a.x,&a.y,&b.x,&b.y,&c.x,&c.y,&tp.x,&tp.y);
         GetCircle( a,b,c );
         printf("Case #%d: ",ca++);
         if( dis( tp,cc.center )<=cc.r ) printf("Danger\n");
         else printf("Safe\n");
     }
     return ;
 }