4.3: Equations as Sentence Parts

• Grammatically, you can think of an equation as a single noun and generally treat it as such. Usually an equation is followed by a comma after it is presented (especially when followed by descriptions of its individual members) or by a period if the sentence has ended. Some journals do not follow the equation with any punctuation marks, relying on the wording of the sentence alone to carry the meaning.
• Short and uncomplicated equations can simply be included as part of a sentence without any special spacing. However, be sure that the equation flows as a readable unit of the sentence. Example:

The equation 2H2+1O2 →2H2O represents how hydrogen and oxygen react to form water.

• It is common to use the word "we" to introduce equations to enhance efficiency, foster readability, and promote the active voice. Handy phrases with which to introduce equations include "we can express," "we can approximate," and "we can describe." Example:

We can express the distance of this transition region by the equation . . .

• If an equation is too long for a single line, break it just before a "verb" (such as the = sign) or a "conjunction" (such as the + sign) and make the following symbol the first member of the next line, then continue the equation.
• When appropriate, define members of the equation just after you present it, usually by introducing them with the word "where." For example, if you wanted to define "t" and "n" just after they appeared in an equation, a phrase such as this would appear: "where t is the film thickness and n is a constant equal to 0.4."
• Unlike a figure or table, an equation does not "stand alone" in a paper. Always use the wording that precedes an equation to introduce it, thus making it a distinct member of a particular sentence.