> solve({y1=a*x1^2+b*x1+c, y2=a*x2^2+b*x2+c, y3=a*x3^2+b*x3+c}, {a,b,c});
                                                              2        2        2           2        2        2
     -x1 y2 + x1 y3 + x3 y2 + x2 y1 - x2 y3 - x3 y1        -x1  y2 + x1  y3 + x3  y2 - y3 x2  + y1 x2  - y1 x3
{a = ----------------------------------------------, b = - ----------------------------------------------------,
                           %1                                                       %1

                 2     2           2           2           2                 2
        -y3 x1 x2  + x1  x2 y3 + x3  x1 y2 - x3  x2 y1 - x1  x3 y2 + y1 x3 x2
    c = ----------------------------------------------------------------------}
                                          %1

            2        2        2        2        2        2
%1 := -x1 x2  + x1 x3  + x3 x2  + x2 x1  - x2 x3  - x3 x1
