Ma trận của dạng toàn phương \[f\left( {{x_1},{x_2},{x_3}} \right) = {x_1}^2 - 2{x_1}{x_2} - {x_1}{x_3}\] là:

\[A = \left( {\begin{array}{*{20}{c}}\begin{array}{l}1\\ - 2\\ - 1\end{array}&\begin{array}{l} - 2\\0\\0\end{array}&\begin{array}{l} - 1\\0\\0\end{array}\end{array}} \right)\]

\[A = \left( {\begin{array}{*{20}{c}}\begin{array}{l}1\\ - 1\\\frac{{ - 1}}{2}\end{array}&\begin{array}{l} - 1\\0\\0\end{array}&\begin{array}{l}\frac{{ - 1}}{2}\\0\\0\end{array}\end{array}} \right)\]

\[A = \left( {\begin{array}{*{20}{c}}\begin{array}{l}\frac{1}{2}\\ - 1\\\frac{{ - 1}}{2}\end{array}&\begin{array}{l} - 1\\0\\0\end{array}&\begin{array}{l}\frac{{ - 1}}{2}\\0\\0\end{array}\end{array}} \right)\]