有向數乘除運算 | Multiplication and Division of Directed Numbers

影片內容：

00:24 – 有向數乘法 | multiplication of directed numbers
03:13 – 點解負負會得正? | why is the product of two negative numbers positive?
07:10 – 有向數除法 | division of directed numbers
09:41 – 例子 | example
10:37 – 除以零點算? | what if zero is the divisor?