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autoreconf --install && ./configure && make
原点n坐标为：(Xn,Yn),半径为Rn,则有：
F(n):  (x - Xn)^2 + (y - Yn)^2 = Rn^2
求两圆相交直线：
F(1) - F(0): 2*(X1-X0)*x + (X0^2-X1^2) + 2*(Y1-Y0)*y + (Y0^2 - Y1^2) = R0^2 - R1^2
                  (R0^2-R1^2)-(X0^2-X1^2)-(Y0^2-Y1^2)     X1 - X0
==>          y = ------------------------------------- - --------- * x
                           2 * (Y1 - Y2)                  Y1 - Y2

                  (R0+R1)(R0-R1) - (X0+X1)(X0-X1) - (Y0+Y1)(Y0-Y1)     X1 - X0
==>          y = -------------------------------------------------- - --------- * x
                           2 * (Y1 - Y2)                               Y1 - Y2
define SUM(X) (X0+X1)
define SUB(X) (X0-X1)
                  SUM(X)*SUB(X) + SUM(Y)*SUB(Y) - SUM(R)*SUB(R)     -SUB(X)
==>          y = ----------------------------------------------- + --------- * x
                           2 * SUB(Y)                                SUB(Y)

==>          y = A*x + B

有三个圆时，可以得到两个上面的式子。很容易就可以求得中心点。