Editing TeX (section)

===Mathematical example===
{{Anchor|math mode|mathematics mode|mathematical mode}}

TeX provides a different text syntax specifically for mathematical formulas. For example, the [[quadratic formula]] (which is the solution of the [[quadratic equation]]) appears as:

{|
! Source code !! Renders as
|-
|<syntaxhighlight lang="latex">
The quadratic formula is $-b \pm \sqrt{b^2 - 4ac} \over 2a$
\bye
</syntaxhighlight>
|<math>\hbox{The quadratic formula is } \textstyle{-b \pm \sqrt{b^2 - 4ac} \over 2a}</math>
|}

The formula is printed in a way a person would write by hand, or typeset the equation. In a document, entering ''mathematics mode'' is done by starting with a $ symbol, then entering a formula in TeX syntax, and closing again with another of the same symbol. Knuth explained in jest that he chose the dollar sign to indicate the beginning and end of mathematical mode in plain TeX because typesetting mathematics was traditionally supposed to be expensive.{{Sfn | Knuth | 1984 | loc = Ch. 16: Typing Math Formulas | p = 127}} ''Display mathematics'' (mathematics presented centred on a new line) is similar but uses $$ instead of a single $ symbol. For example, the above with the quadratic formula in display math:

{|
! Source code !! Renders as
|-
|<syntaxhighlight lang="latex">
The quadratic formula is $$-b \pm \sqrt{b^2 - 4ac} \over 2a$$
\bye
</syntaxhighlight>
|<math>\begin{matrix} \text{The quadratic formula is} \\  \displaystyle{\frac{-b \pm \sqrt{b^2 - 4ac}}{2a}} \end{matrix}</math>
|}

(The examples here are not actually rendered with TeX; spacing, character sizes, and all else may differ.)