ex2.tex

\input arrows.tex
 
$$\vcenter {\offinterlineskip
   \def \cc#1{ \hfil #1 \hfil }
   \halign {
   % preamble
      \cc {$#$}& #\tv && \cc {$#$}
   \cr
      x&& -\infty && 3/7
   \cr
   \noalign {\hrule height 1pt }
      3-7x&& & +& 0
   \cr
   \noalign {\hrule }
      -7&& & -
   \cr
   \noalign {\hrule height 1pt}
      f' (x)&& & -& \doublevrule 
   \cr
   \noalign {\hrule height 1pt}
      \bucenter {$f (x)$}&&
      \bup {\phantom {1}} &
      \brightdownarrow & \doublevrule
   \cr
}}
$$
\bye