考点梳理
1. 乘法规则
(1)行矩阵[a11 a12]与列矩阵\o(\s\up7(b11b21的乘法规则：
[a11 a12]\o(\s\up7(b11b21＝[a11×b11＋a12×b21]．
(2)二阶矩阵\o(\s\up7(a11a21与列向量\o(\s\up7(x0y0的乘法规则：
\o(\s\up7(a11a21 \o(\s\up7(x0y0＝\o(\s\up7(a11×x0＋a12×y0a21×x0＋a22×y0.
(3)两个二阶矩阵相乘的结果仍然是一个矩阵，其乘法法则如下：
\o(\s\up7(a11a21 \o(\s\up7(b11b21＝\o(\s\up7(a11×b11＋a12×b21a21×b11＋a22×b21
(4)两个二阶矩阵的乘法满足结合律，但不满足交换律和消去律．即(AB)C＝A(BC)，AB≠BA，由AB＝AC不一定能推出B＝C.
