平面坐标A=sqrt((Xa-Xb)*(Xa-Xb)+(Ya-Yb)*(Ya-Yb))B=sqrt((Xa-Xc)*(Xa-Xc)+(Ya-Yc)*(Ya-Yc))C=sqrt((Xb-Xc)*(Xb-Xc)+(Yb-Yc)*(Yb-Yc))y=(A+B+C)/2x=sqrt(y*(y-A)*(y-B)*(y-C))r=A*B*C/(4*x)