设过\(P_i(x_i,y_i),i=1,2,3\)的圆的方程为
\begin{equation}
x^2+y^2+Ax+By+C=0.\tag{3.4.1}
\end{equation}
将
\(P_i(x_i,y_i),i=1,2,3\)分别代入
(3.4.1)得到关于未知量
\(A,B,C\)的线性方程组
\begin{equation}
\left\{\begin{array}{ccl}
Ax_1+By_1+C&=&-(x_1^2+y_1^2),\\
Ax_2+By_2+C&=&-(x_2^2+y_2^2),\\
Ax_3+By_3+C&=&-(x_3^2+y_3^2).\\
\end{array}\right.\tag{3.4.2}
\end{equation}
\begin{equation*}
\begin{vmatrix}
x_1&y_1&1\\
x_2&y_2&1\\
x_3&y_3&1
\end{vmatrix}\neq 0.
\end{equation*}
\begin{equation*}
A=\frac{\begin{vmatrix}
-(x_1^2+y_1^2)&y_1&1\\
-(x_2^2+y_2^2)&y_2&1\\
-(x_3^2+y_3^2)&y_3&1
\end{vmatrix}}{\begin{vmatrix}
x_1&y_1&1\\
x_2&y_2&1\\
x_3&y_3&1
\end{vmatrix}},
\end{equation*}
\begin{equation*}
B=\frac{\begin{vmatrix}
x_1&-(x_1^2+y_1^2)&1\\
x_2&-(x_2^2+y_2^2)&1\\
x_3&-(x_3^2+y_3^2)&1
\end{vmatrix}}{\begin{vmatrix}
x_1&y_1&1\\
x_2&y_2&1\\
x_3&y_3&1
\end{vmatrix}},
\end{equation*}
\begin{equation*}
C=\frac{\begin{vmatrix}
x_1&y_1&-(x_1^2+y_1^2)\\
x_2&y_2&-(x_2^2+y_2^2)\\
x_3&y_3&-(x_3^2+y_3^2)
\end{vmatrix}}{\begin{vmatrix}
x_1&y_1&1\\
x_2&y_2&1\\
x_3&y_3&1
\end{vmatrix}}.
\end{equation*}
\begin{equation*}
\begin{array}{cl}
(x^2+y^2)\begin{vmatrix}
x_1&y_1&1\\
x_2&y_2&1\\
x_3&y_3&1
\end{vmatrix}+x\begin{vmatrix}
-(x_1^2+y_1^2)&y_1&1\\
-(x_2^2+y_2^2)&y_2&1\\
-(x_3^2+y_3^2)&y_3&1
\end{vmatrix}&\\
+y\begin{vmatrix}
x_1&-(x_1^2+y_1^2)&1\\
x_2&-(x_2^2+y_2^2)&1\\
x_3&-(x_3^2+y_3^2)&1
\end{vmatrix}+\begin{vmatrix}
x_1&y_1&-(x_1^2+y_1^2)\\
x_2&y_2&-(x_2^2+y_2^2)\\
x_3&y_3&-(x_3^2+y_3^2)
\end{vmatrix}&=0,
\end{array}
\end{equation*}
\begin{equation*}
\begin{array}{cl}
(x^2+y^2)\begin{vmatrix}
x_1&y_1&1\\
x_2&y_2&1\\
x_3&y_3&1
\end{vmatrix}-x\begin{vmatrix}
x_1^2+y_1^2&y_1&1\\
x_2^2+y_2^2&y_2&1\\
x_3^2+y_3^2&y_3&1
\end{vmatrix}&\\
+y\begin{vmatrix}
x_1^2+y_1^2&x_1&1\\
x_2^2+y_2^2&x_2&1\\
x_3^2+y_3^2&x_3&1
\end{vmatrix}-\begin{vmatrix}
x_1^2+y_1^2&x_1&y_1\\
x_2^2+y_2^2&x_2&y_2\\
x_3^2+y_3^2&x_3&y_3
\end{vmatrix}&=0.
\end{array}
\end{equation*}
因此所求圆的方程为
\begin{equation*}
\begin{vmatrix}
x^2+y^2&x&y&1\\
x_1^2+y_1^2&x_1&y_1&1\\
x_2^2+y_2^2&x_2&y_2&1\\
x_3^2+y_3^2&x_3&y_3&1
\end{vmatrix}=0.
\end{equation*}