zeta=(2*x1*y-x1*y2-x1*y3+y1*x2+y2*x3+y1*x3-2*y1*x-2*x3*y+2*x*y3-x2*y3)/(-x1*y3-y1*x2+x2*y3+y1*x3+x1*y2-y2*x3);
eta=-1/(-x1*y3-y1*x2+x2*y3+y1*x3+x1*y2-y2*x3)*(2*x1*y-x1*y2-x1*y3+y1*x3-2*x2*y+x2*y3-y2*x3-2*y1*x+y1*x2+2*y2*x);

S = fabs(-x1*(y3-y2)+x2*y3-x3*y2+(x3-x2)*y1 )*0.5;