【求助】帮忙解释下函数的思想或者思路

[yangenshui]

%circfit.m

function [xc,yc,R,a] = circfit(x,y)
%CIRCFIT Fits a circle in x,y plane
% [XC, YC, R, A] = CIRCFIT(X,Y)
% Result is center point (yc,xc) and radius R.A is an

% optional output describing the circle's equation:
% x^2+y^2+a(1)*x+a(2)*y+a(3)=0

n=length(x);
xx=x.*x;
yy=y.*y;
xy=x.*y;
A=[sum(x) sum(y) n;sum(xy) sum(yy)...
sum(y);sum(xx) sum(xy) sum(x)];
B=[-sum(xx+yy) ; -sum(xx.*y+yy.*y) ; -sum(xx.*x+xy.*y)];
a=A\B;
xc = -.5*a(1);
yc = -.5*a(2);
R = sqrt((a(1)^2+a(2)^2)/4-a(3));
这是一个圆的曲线拟合，网上摘下来的，但具体的思想、思路不清楚，哪位高手给
解释解释啊？
谢谢！！