線性代數的第一堂課──矩陣乘法的定義

https://ccjou.wordpress.com/2010/06/18/%E7%B7%9A%E6%80%A7%E4%BB%A3%E6%95%B8%E7%9A%84%E7%AC%AC%E4%B8%80%E5%A0%82%E8%AA%B2-%E7%9F%A9%E9%99%A3%E4%B9%98%E6%B3%95%E7%9A%84%E5%AE%9A%E7%BE%A9/