#include 
#include 
#include 

void main() {
    double x1,x2,x3;
    double y1,y2,y3;
    double x12, x22, x32, y12, y22, y32;
    double a, b, r2, r;

    x1 = 9.5418;
    y1 = 17.5771;
    x2 = 14.1421;
    y2 = -14.1421;
    x3 = -20;
    y3 = 0;
    x12 = x1*x1; x22 = x2*x2; x32 = x3*x3;
    y12 = y1*y1; y22 = y2*y2; y32 = y3*y3;

    a= ((x12-x32+y12-y32)/(2*y1-2*y3)-(x12-x22+y12-y22)/(2*y1-2*y2)) / ((x2-x1)/(y1-y2)-(x3-x1)/(y1-y3));
    b = (2*a*(x2-x1)+x12-x22+y12-y22)/(2*y1-2*y2);
    r2 = (x1-a)*(x1-a)+(y1-b)*(y1-b);
    r = sqrt(r2)

    printf("%f, %f, %f\n", a, b, r);
}

なぜか、これで sqrt 文のところでコンパイルエラーが発生。
r2が負の値になるはずもなく、一応、r2の中身を確認したけど問題なし。
なぜだー。