Doctor Ian 要說的正是我想要表答的意思：

The original expression, without the extra parentheses, is a little
like this sentence: "Bob and Alice were drunk, so we took their keys
and drove them home." Most people would 'know' that the speaker drove
Alice and Bob home; but the rules of English say that the speaker
drove the keys home, leaving us in the dark about what happened to
Alice and Bob.

This kind of ambiguity is largely ignored in everyday language, where
semantic information is sufficient for disambiguation. However, in
math, numbers (like '2') have no semantic information associated with
them, so it becomes essential to know what the rules are, and to
follow them.

2/ab = 2/(ab) 只是一種積非成是的誤用，這和數學定義是相衝突的。因為數學上找不到這樣的定義。
不過因為很多人這樣用，包含大師級人物，所以大家就理所當然了。

大師不代表就不會寫錯字，大師厲害不是因為他寫的字總是對的才叫大師。奶油酫大的例子也表示，即使大師的表示方法也自己產生了 2/ab = 2/(ab) 或 2/ab = (2/a)b 的矛盾。

不過講這麼多，怎不寫信去問問大師的看法？