🧩 矩阵乘法
\(\mathbf{C} = \mathbf{A} \times \mathbf{B}\)，新矩阵的元素由行×列点积得到：
📘 例子：
\(\mathbf{A} = \begin{bmatrix}2&1\\3&4\end{bmatrix}\) \(\mathbf{B} = \begin{bmatrix}5&6\\7&8\end{bmatrix}\)
\(C_{00} = 2×5+1×7 = 17\) \(C_{01}=2×6+1×8=20\)
\(C_{10} = 3×5+4×7 = 43\) \(C_{11}=3×6+4×8=50\)
➡️ \(\mathbf{C} = \begin{bmatrix}17&20\\43&50\end{bmatrix}\)