3点を通る円

求める円の方程式を
\[ x^{2}+y^{2}+ax+by+c=0 \] とする。
このとき、3点A,B,Cは円上にあるのでこの方程式を満たし、
\[ \left(\begin{array}{c} x_{1}^{\;2}+y_{1}^{\;2}\\ x_{2}^{\;2}+y_{2}^{\;2}\\ x_{3}^{\;2}+y_{3}^{\;2} \end{array}\right)+\left(\begin{array}{ccc} x_{1} & y_{1} & 1\\ x_{2} & y_{2} & 1\\ x_{3} & y_{3} & 1 \end{array}\right)\left(\begin{array}{c} a\\ b\\ c \end{array}\right)=0 \] となるので、
\begin{align*} \left(\begin{array}{c} a\\ b\\ c \end{array}\right) & =-\left(\begin{array}{ccc} x_{1} & y_{1} & 1\\ x_{2} & y_{2} & 1\\ x_{3} & y_{3} & 1 \end{array}\right)^{-1}\left(\begin{array}{c} x_{1}^{\;2}+y_{1}^{\;2}\\ x_{2}^{\;2}+y_{2}^{\;2}\\ x_{3}^{\;2}+y_{3}^{\;2} \end{array}\right)\\ & =-\frac{1}{x_{1}y_{2}+y_{1}x_{3}+x_{2}y_{3}-x_{1}y_{3}-y_{1}x_{2}-y_{2}x_{3}}\left(\begin{array}{ccc} y_{2}-y_{3} & y_{3}-y_{1} & y_{1}-y_{2}\\ x_{3}-x_{2} & x_{1}-x_{3} & x_{2}-x_{1}\\ x_{2}y_{3}-y_{2}x_{3} & y_{1}x_{3}-x_{1}y_{3} & x_{1}y_{2}-y_{1}x_{2} \end{array}\right)\left(\begin{array}{c} x_{1}^{\;2}+y_{1}^{\;2}\\ x_{2}^{\;2}+y_{2}^{\;2}\\ x_{3}^{\;2}+y_{3}^{\;2} \end{array}\right) \end{align*} となる。
これより円の方程式は、
\begin{align*} 0 & =x^{2}+y^{2}+ax+by+c\\ & =x^{2}+y^{2}+\left(\begin{array}{ccc} x & y & 1\end{array}\right)\left(\begin{array}{c} a\\ b\\ c \end{array}\right)\\ & =x^{2}+y^{2}-\frac{1}{x_{1}y_{2}+y_{1}x_{3}+x_{2}y_{3}-x_{1}y_{3}-y_{1}x_{2}-y_{2}x_{3}}\left(\begin{array}{ccc} x & y & 1\end{array}\right)\left(\begin{array}{ccc} y_{2}-y_{3} & y_{3}-y_{1} & y_{1}-y_{2}\\ x_{3}-x_{2} & x_{1}-x_{3} & x_{2}-x_{1}\\ x_{2}y_{3}-y_{2}x_{3} & y_{1}x_{3}-x_{1}y_{3} & x_{1}y_{2}-y_{1}x_{2} \end{array}\right)\left(\begin{array}{c} x_{1}^{\;2}+y_{1}^{\;2}\\ x_{2}^{\;2}+y_{2}^{\;2}\\ x_{3}^{\;2}+y_{3}^{\;2} \end{array}\right) \end{align*} となる。