Stacked vs Unstacked

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Stacking of fractions (ie, separating numerator and denominator by a horizontal line) should be avoided in favor of “unstacking” (ie, using a slash in place of the horizontal line) unless this sacrifices clarity (see 8.4.4, Punctuation, Forward Slash [Virgule, Solidus], In Equations).$$y=\left(x_1+x_2\right)/\left(x_1-x_2\right)\text{instead of}y=\frac{x_1+x_2}{x_1-x_2}$$

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 $$a+\frac{b+c}{d}+e,$$

if written as a + b + c/d + e, is ambiguous and could have several interpretations, such as $$\frac{a+b+c}{d+e}$$ or $$a+b+\frac{c}{d}+e.$$

The expression’s meaning is unambiguous if set off as follows: $$a+\left[\left(b+c\right)/d\right]+e.$$

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 In Formulas in 8.5.2, Punctuation, Parentheses and Brackets, Brackets). All fences should be present in matched pairs.$$\left\{\left[\left(a+b\right)/c\right]+\left[\left(d+e\right)/f\right]\right\}+g$$