ex6.tex

\input arrows.tex
 
$$\vcenter {\offinterlineskip
   \def \cc#1{ \hfil #1 \hfil }
   \halign {
   % preamble
      \cc {$#$}& #\tv && \cc {$#$}
   \cr
      x&& -3&& -1&& 1&& 3&
   \cr
   \noalign {\hrule height 1pt}
      \buucenter {$f (x)$}&& \down {?}&
      \brightuuparrow & \buup {$3$}&
      \brightddownarrow & \down{$-1$}& 
      \brightuuparrow & \buup {?}
   \cr
}}
$$
\bye