AMA Manual of Style - Stacy L. Christiansen, Cheryl Iverson 2020
Stacked vs Unstacked Fractions or Formulas
Both stacking of fractions (ie, separating numerator and denominator by a horizontal line) and unstacking of fractions (ie, using a slash in place of the horizontal line) are acceptable as long as clarity is not lost (see 8.4.4, Forward Slash [Virgule, Solidus], In Equations).
Whenever a fraction is unstacked, parentheses, brackets, and braces (collectively called “fences” in mathematical notation) should be used as appropriate to avoid ambiguity. For instance, the expression
if written as a + b + c/d + e, is ambiguous and could have several interpretations, such as
The expression’s meaning is unambiguous if presented as follows:
Parentheses should be used to set off simple expressions. If additional fences are needed for clarity, parenthetical expressions should be set off in brackets, and bracketed expressions should be set off with braces. Note that parentheses are thus always the innermost fences (see 184.108.40.206, Brackets, In Formulas). All fences should be present in matched pairs.