ex3.tex

\input arroaws.tex
 
$$\vcenter {\offinterlineskip
   \def \cc#1{ \hfil #1 \hfil }
   \halign {
   % preamble
      \cc {$#$}& #\tv && \cc {$#$}
   \cr
      x&& 0&&& e^{1/2}&& +\infty
   \cr
   \noalign {\hrule height 1pt }
      1 - 2\ln x&& && +& 0& -
   \cr
   \noalign {\hrule }
      x^2&& 0&& +& \tv & +
   \cr
   \noalign {\hrule height 1pt}
      f' (x)&& \doublevrule && +& 0& -
   \cr
   \noalign {\hrule height 1pt}
      \buucenter {$f (x)$}&&
      \doublevrule &
      \down {$-\infty $}&
      \brightuuparrow & \buup {$2(e^{-1/2}-1) \approx -0, 79$}& 
      \brightddownarrow & \down {$-2$}
   \cr
}}
$$
\bye