Rudy's OBTF

Boxing

In equation, use \boxed{...}:

\begin{equation*}
  \boxed{x} = 1
\end{equation*}

\begin{equation*} \boxed{x} = 1 \end{equation*}

but in align, use \Aboxed{...} from mathtools:

\begin{align*}
  x & = 1 \\
  \Aboxed{y & = 2}
\end{align*}

\begin{align*} x & = 1 \\ \Aboxed{y & = 2} \end{align*}