In algebra, multiplication involving variables is often written as a juxtaposition (e.g. xy for x times y or 5x for five times x). This notation can also be used for quantities that are surrounded by parentheses (e.g. 5(2) or (5)(2) for five times two).
明明就有寫....
乘號只能以定義中列舉的方式省略。所以 ab = (a x b) 的爭議在於定義上無此表示法。
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