%%%%%%texsample.tex
\newif\iftth
\input docurl
\overfullrule0pt
\newcount\probno\probno0
\def\advprob{\advance\probno by1\null}
\magnification=1200
\vsize=9.5truein
\hsize=6.5truein
\newif\iftth
\long\def\tthdump#1{#1}
 \tthdump{\footline{\tt \hfill \jobname.tex}}

We like to write equations and formulas which can be read by others:

 $$\eqalign{ A& =\pm\int_{a}^b f(x)\, dx\cr
              & =\cases{\int_{a}^b f(x)\, dx, & $f(x) \geq 0$,\cr
                       -\int_{a}^b f(x)\, dx, & $f(x) \leq 0$. }\cr }$$

\bye