Final answer:
In mathematics, parentheses are generally used around variables to denote explicitly that the enclosed operations should be conducted first. So, if the variable 'a' is representing a simplified expression or a combination of constants, it will typically be enclosed in parentheses to indicate this. If referring to the dimensions of a physical quantity, square brackets are used.
Step-by-step explanation:
The question at hand is deciding which symbol, bracket or parenthesis, should be placed around the variable a when simplifying or representing a mathematical expression or equation. In mathematics, parentheses are typically used to indicate that the operations within should be completed first, according to the order of operations. Brackets are often used to clarify which parts of an expression are included within the scope of a calculation or for specifying the dimensions of physical quantities.
If you were to gather constant quantities inside the parentheses to replace these constants with a single symbol a, the use of parentheses would be appropriate. For example, in a simplified algebraic expression like (3 + 5)a, the terms within the parentheses should be computed first before being multiplied by a. Likewise, in physics, if stating the dimensions of a quantity such as [a] = ML-³, square brackets are used to indicate the dimensions of a in terms of mass (M) and length (L).
Therefore, if we are talking about simplifying an expression or replacing constants with a single variable a, it is likely that parentheses would be necessary, as they denote that the expression within is treated as a unit. On the other hand, if we are discussing the dimensions of a physical quantity represented by a, square brackets would be used.