抄的外心模版。然后，输出认真一点。1Y。
1 #include <cstdio> 2 #include <cstring> 3 #include <string> 4 #include <cmath> 5 #include <algorithm> 6 using namespace std; 7 #define eps 1e-8 8 struct point 9 { 10 double x,y; 11 }; 12 struct line 13 { 14 point a,b; 15 }; 16 point intersection(line u,line v) 17 { 18 point ret = u.a; 19 double t = ((u.a.x-v.a.x)*(v.a.y-v.b.y)-(u.a.y-v.a.y)*(v.a.x-v.b.x)) 20 /((u.a.x-u.b.x)*(v.a.y-v.b.y)-(u.a.y-u.b.y)*(v.a.x-v.b.x)); 21 ret.x += (u.b.x-u.a.x)*t; 22 ret.y += (u.b.y-u.a.y)*t; 23 return ret; 24 } 25 point circumcenter(point a,point b,point c) 26 { 27 line u,v; 28 u.a.x = (a.x+b.x)/2; 29 u.a.y = (a.y+b.y)/2; 30 u.b.x = u.a.x - a.y + b.y; 31 u.b.y = u.a.y + a.x - b.x; 32 v.a.x = (a.x+c.x)/2; 33 v.a.y = (a.y+c.y)/2; 34 v.b.x = v.a.x - a.y + c.y; 35 v.b.y = v.a.y + a.x - c.x; 36 return intersection(u,v); 37 } 38 double dis(point p1,point p2) 39 { 40 return sqrt((p1.x-p2.x)*(p1.x-p2.x) + (p1.y-p2.y)*(p1.y-p2.y)); 41 } 42 int main() 43 { 44 point p1,p2,p3,cr; 45 double r,c,d,e; 46 while(scanf("%lf%lf",&p1.x,&p1.y)!=EOF) 47 { 48 scanf("%lf%lf",&p2.x,&p2.y); 49 scanf("%lf%lf",&p3.x,&p3.y); 50 cr = circumcenter(p1,p2,p3); 51 r = dis(cr,p1); 52 cr.x = -cr.x; 53 cr.y = -cr.y; 54 if(cr.x > -eps) 55 printf("(x + %.3f)^2",cr.x+eps); 56 else 57 printf("(x - %.3f)^2",-(cr.x+eps)); 58 if(cr.y > -eps) 59 printf(" + (y + %.3lf)^2 = ",cr.y+eps); 60 else 61 printf(" + (y - %.3lf)^2 = ",-(cr.y+eps)); 62 printf("%.3lf^2 ",r+eps); 63 c = cr.x*2; 64 d = cr.y*2; 65 e = cr.x*cr.x + cr.y*cr.y - r*r; 66 printf("x^2 + y^2 "); 67 if(c > -eps) 68 printf("+ %.3fx ",c+eps); 69 else 70 printf("- %.3fx ",-(c+eps)); 71 if(d > -eps) 72 printf("+ %.3fy ",d+eps); 73 else 74 printf("- %.3fy ",-(d+eps)); 75 if(e > -eps) 76 printf("+ %.3f = 0 ",e+eps); 77 else 78 printf("- %.3f = 0 ",-(e+eps)); 79 } 80 }