二次型 $f\left(x_1, x_2, x_3\right)=x_1^2+2 x_2^2+3 x_3^2+2 x_1 x_2+2 x_1 x_3$ 的矩阵为
A
$\left(\begin{array}{lll}1 & 1 & 1 \\ 1 & 2 & 0 \\ 1 & 0 & 3\end{array}\right)$
B
$\left(\begin{array}{lll}1 & 2 & 2 \\ 2 & 2 & 0 \\ 2 & 0 & 3\end{array}\right)$
C
$\left(\begin{array}{lll}1 & 1 & 0 \\ 1 & 2 & 1 \\ 0 & 1 & 3\end{array}\right)$
D
$\left(\begin{array}{lll}1 & 0 & 0 \\ 0 & 2 & 0 \\ 0 & 0 & 3\end{array}\right)$
E
F