\begin{minipage}{0.5\textwidth}
\begin{tabbing}
$f$ \= $= (a\land d\land c)\lor(b\land\lnot d\land\lnot a)\lor(c \implies \lnot d) \lor (a \implies \lnot b)$\\
	\>$ f_b $ \= $= \top$ \\
	\>$ f_{\lnot b} =\top$ \\
\end{tabbing}
\end{minipage}
\begin{minipage}{0.5\textwidth}
\begin{center}
\begin{forest}
for tree={circle, draw, no edge,
            minimum size=2em,
            inner sep=0pt,
            s sep=5mm,
            l sep=5mm}
[$f$, rectangle, draw,  tikz={\draw [line] () to (0,-1);}
]
\end{forest}
\end{center}

\end{minipage}