If you prefer to use Plain TeX, here is the same example in that notation.
% math.tex: Sample of TeX math for inclusion in DocBook
%
\hsize=6in
\nopagenumbers
$$\int \tanh^{-1}{x\over a}dx =
x \tanh^{-1}{x\over a}+{a\over 2}\log(a^2-x^2),\qquad
\left(\left| x\over a \right| < 1\right)
\leqno{(14)}
$$
\bye
The line \hsize=6in sets the text
column width to six inches.
Because the production process wraps a box around
whatever text is on the page, use \nopagenumbers to turn off page numbering.
To place equation numbers on the left, use \leqno or \leqalignno.