9.  Displayed Equations

      To tell EQN where a mathematical expression begins and ends, we mark it with lines beginning .EQ and .EN. Thus if you type the lines ^EQ
x=y+z
^EN
your output will look like

The .EQ and .EN are copied through untouched; they are not otherwise processed by EQN. This means that you have to take care of things like centering, numbering, and so on yourself. The most common way is to use the TROFF and NROFF macro package package `-ms' developed by M. E. Lesk[3], which allows you to center, indent, left-justify and number equations.

      With the `-ms' package, equations are centered by default. To left-justify an equation, use .EQ L instead of .EQ. To indent it, use .EQ I. Any of these can be followed by an arbitrary `equation number' which will be placed at the right margin. For example, the input ^EQ I (3.1a)
x = f(y/2) + y/2
^EN
produces the output

      There is also a shorthand notation so in-line expressions like can be entered without .EQ and .EN. We will talk about it in section 19.