Language: C

double x[3],y[3],a,b,c,A,B,C,X,Y,r;
main(n,i,j,k){
	i=0,j=1,k=2;
	for(scanf("%d",&n);n--;printf("%.3f %.3f %.3f\n",x,y,r)){
		scanf("%lf%lf%lf%lf%lf%lf",x,y,x+1,y+1,x+2,y+2);
		a=2*x[j]-2*x[i],b=2*y[j]-2*y[i],c=x[i]*x[i]-x[j]*x[j]+y[i]*y[i]-y[j]*y[j];
		A=2*x[k]-2*x[i],B=2*y[k]-2*y[i],C=x[i]*x[i]-x[k]*x[k]+y[i]*y[i]-y[k]*y[k];
		x=(b*C-B*c)/(a*B-A*b),y=(c*A-C*a)/(a*B-A*b);
		r=hypot(x[i]-x,y[i]-y);
	}
exit(0);}

Compiler output:
main.c:2:1: error: return type defaults to 'int' [-Wimplicit-int]
    2 | main(n,i,j,k){
      | ^~~~
main.c: In function 'main':
main.c:2:1: error: type of 'n' defaults to 'int' [-Wimplicit-int]
main.c:2:1: error: type of 'i' defaults to 'int' [-Wimplicit-int]
main.c:2:1: error: type of 'j' defaults to 'int' [-Wimplicit-int]
main.c:2:1: error: type of 'k' defaults to 'int' [-Wimplicit-int]
main.c:4:13: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
    4 |         for(scanf("%d",&n);n--;printf("%.3f %.3f %.3f\n",x,y,r)){
      |             ^~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | double x[3],y[3],a,b,c,A,B,C,X,Y,r;
main.c:4:13: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
    4 |         for(scanf("%d",&n);n--;printf("%.3f %.3f %.3f\n",x,y,r)){
      |             ^~~~~
main.c:4:13: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:4:32: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
    4 |         for(scanf("%d",&n);n--;printf("%.3f %.3f %.3f\n",x,y,r)){
      |                                ^~~~~~
main.c:4:32: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:4:32: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:4:32: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:8:18: error: assignment to expression with array type
    8 |                 x=(b*C-B*c)/(a*B-A*b),y=(c*A-C*a)/(a*B-A*b);
      |                  ^
main.c:8:40: error: assignment to expression with array type
    8 |                 x=(b*C-B*c)/(a*B-A*b),y=(c*A-C*a)/(a*B-A*b);
      |                                        ^
main.c:9:19: error: implicit declaration of function 'hypot' [-Wimplicit-function-declaration]
    9 |                 r=hypot(x[i]-x,y[i]-y);
      |                   ^~~~~
main.c:1:1: note: include '<math.h>' or provide a declaration of 'hypot'
  +++ |+#include <math.h>
    1 | double x[3],y[3],a,b,c,A,B,C,X,Y,r;
main.c:9:19: warning: incompatible implicit declaration of built-in function 'hypot' [-Wbuiltin-declaration-mismatch]
    9 |                 r=hypot(x[i]-x,y[i]-y);
      |                   ^~~~~
main.c:9:19: note: include '<math.h>' or provide a declaration of 'hypot'
main.c:9:29: error: invalid operands to binary - (have 'double' and 'double *')
    9 |                 r=hypot(x[i]-x,y[i]-y);
      |                         ~~~~^
      |                          |  |
      |                          |  double *
      |                          double
main.c:9:36: error: invalid operands to binary - (have 'double' and 'double *')
    9 |                 r=hypot(x[i]-x,y[i]-y);
      |                                ~~~~^
      |                                 |  |
      |                                 |  double *
      |                                 double
main.c:11:1: error: implicit declaration of function 'exit' [-Wimplicit-function-declaration]
   11 | exit(0);}
      | ^~~~
main.c:1:1: note: include '<stdlib.h>' or provide a declaration of 'exit'
  +++ |+#include <stdlib.h>
    1 | double x[3],y[3],a,b,c,A,B,C,X,Y,r;
main.c:11:1: warning: incompatible implicit declaration of built-in function 'exit' [-Wbuiltin-declaration-mismatch]
   11 | exit(0);}
      | ^~~~
main.c:11:1: note: include '<stdlib.h>' or provide a declaration of 'exit'