Maths equations, as easy as pi
Parts of equations and when to italicise, including the variable n for sample size
Many editors will come across maths equations in their work. Knowing what the parts of an equation are, and whether to italicise the letters within the equation, will simplify the editing process and enable you to use correct terminology when writing comments to the author.
Parts of an equation
There are several parts to an algebraic equation. Take this equation:
The entire line of text is called an equation.
A number by itself is a constant. Constants don’t change their values. In this equation, the numbers 5 and 1 are constants (they will keep the values of 5 and 1 as the equation is solved).
A variable is a letter in an equation that can be substituted for different values. It represents an unknown value. In this equation, x is a variable.
A coefficient is a number located next to a variable. In this equation, 3 is a coefficient (because it is next to the variable x).
The combination of a variable and a coefficient is a term. In this equation, 3x is a term.
An expression is a combination of terms. In this equation, all the items before the equals sign make up an expression.
Italicising variables (and sometimes constants)
You may have noticed that the variables in the equation above are italicised. Most style guides support the convention of italic variables. This practice is said to have come from typesetting conventions, where an italic font helps to distinguish variables from other mathematical objects, such as a multiplication sign.
Variables are usually italicised even if they stand for something. For example, the equation for the area (a) of a triangle uses the variables l (length) and h (height).
a = lh
Single-letter symbols for constants are also usually italicised. For example, c = the speed of light in a vacuum and k = the Boltzmann constant.
The one everyone asks about, n for sample size
In equations dealing with sample sizes, capital N stands for the total number of a group (the population size), and lower-case n stands for a portion of the total group (the sample size or a subset of the sample).
Both N and n should be italicised.
Mathematical operators and constants
Mathematical operators, including abbreviations such as sin and cos, are usually in Roman font and are not italicised. Neither is pi, whether used as a word or a symbol (π).
It is possible to combine variables with operators and constants. For example, the equation for the circumference of a circle uses the variables C (circumference) and d (diameter), and the constant π.
C = πd
In trigonometry, the sine function uses the variable x.
sin x
