急求：已知空间三点坐标（字母参数表示），如何求出其外接圆的圆心？
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syms x1 x2 x3 y1 y2 y3
S=solve('y-(y1+y2)/2+(x2-x1)/(y2-y1)*(x-(x1+x2)/2)=0','y-(y1+y3)/2+(x3-x1)/(y3-y1)*(x-(x1+x3)/2)=0',x,y)
S.x
ans =
1/2*(y2*x1^2-x1^2*y3+y3*y2^2+y3*x2^2+y1*y3^2-y1*x2^2+y1*x3^2-y1*y2^2-y1^2*y3-x3^2*y2-y3^2*y2+y1^2*y2)/(-y2*x3+y2*x1+y1*x3+x2*y3-x2*y1-x1*y3)
>> S.y
ans =
-1/2*(-x2*x3^2+x2*x1^2+x2^2*x3-y1^2*x3+y2^2*x3-y2^2*x1-x2^2*x1-x1^2*x3+x1*x3^2+x2*y1^2-x2*y3^2+x1*y3^2)/(-y2*x3+y2*x1+y1*x3+x2*y3-x2*y1-x1*y3)