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\begin{tikzpicture}[every tree node/.style={draw,circle},sibling distance=.15cm]
\Tree
[.$\exists x$
[.$\forall y$
[.$\imp$
[.$P$ $x$ $y$ ]
[.$\lor$
[.$Q$ $x$ $y$ ]
[.$R$ $x$ $y$ ]
]
]
]
]
\end{tikzpicture}
\begin{multicols}{2}
$$x=a \land y=a$$
\begin{tikzpicture}[every tree node/.style={draw,circle},sibling distance=.15cm]
\Tree
% [.$\exists x$
% [.$\forall y$
[.$\imp$
[.$P$ $a$ $a$ ]
[.$\lor$
[.$Q$ $a$ $a$ ]
[.$R$ $a$ $a$ ]
]
]
% ]
% ]
\end{tikzpicture}
\begin{align*}
&P(a,a) \imp (Q(a,a) \lor R(a,a)) = \\
&\true \imp (\true \lor \true) = \true
\end{align*}
\columnbreak
$$x=a \land y=b$$
\begin{tikzpicture}[every tree node/.style={draw,circle},sibling distance=.15cm]
\Tree
% [.$\exists x$
% [.$\forall y$
[.$\imp$
[.$P$ $a$ $b$ ]
[.$\lor$
[.$Q$ $a$ $b$ ]
[.$R$ $a$ $b$ ]
]
]
% ]
% ]
\end{tikzpicture}
\begin{align*}
&P(a,b) \imp (Q(a,b) \lor R(a,b)) = \\
&\true \imp (\true \lor \false) = \true
\end{align*}
\end{multicols}
\begin{centering}
$$x=a \land y=c$$
\begin{tikzpicture}[every tree node/.style={draw,circle},sibling distance=.15cm]
\Tree
% [.$\exists x$
% [.$\forall y$
[.$\imp$
[.$P$ $a$ $c$ ]
[.$\lor$
[.$Q$ $a$ $c$ ]
[.$R$ $a$ $c$ ]
]
]
% ]
% ]
\end{tikzpicture}
\begin{align*}
&P(a,c) \imp (Q(a,c) \lor R(a,c)) = \\
&\false \imp (\true \lor \true) = \true
\end{align*}
\end{centering}
We have found an $x$, such that for all $y$ the formula evaluates to \textit{true}. Therefore $$M \models \varphi.$$