ex1.tex

\input arrows.tex
 
$$\vcenter {\offinterlineskip 
   \def \cc#1{ \hfil #1 \hfil }
   \halign {
   % preamble
      \cc {$#$}& #\tv && \cc {$#$}
   \cr
      x&& 0&& 2&& +\infty
   \cr
   \noalign {\hrule height 1pt }
      x-2&& & -& 0& +
   \cr
   \noalign {\hrule }
      x&& 0& +& \tv & +
   \cr
   \noalign {\hrule height 1pt}
      f' (x)&& \doublevrule & -& 0& +
   \cr
   \noalign {\hrule height 1pt}
      \buucenter {$f (x)$}&& 
      \doublevrule &
      \brightddownarrow & \down{$\matrix {1-4\ln 2 \cr \approx -1, 77\cr }$}& 
      \brightuuparrow & \buup {\phantom {1}}
   \cr
}}
$$
 
\bye