x1=0;y1=0;x2=1;y2=2;x3=2;y3=1;a=sqrt((x1-x2)^2+(y1-y2)^2);b=sqrt((x2-x3)^2+(y2-y3)^2);c=sqrt((x3-x1)^2+(y3-y1)^2);r=a*b*c/sqrt((a+b+c)*(a+b-c)*(a-b+c)*(-a+b+c))
