[Tex/LaTex] Referencing parts of equations within a tikz picture

tikz-pgf

I have an equation in a tikz environment and I would like to be able to highlight part of it. For example the following:

\documentclass[12pt]{article}
\usepackage{amsmath,amssymb,latexsym}
\usepackage{tikz}


\begin{document}


\tikzstyle{block} = [draw,fill=blue!60,minimum width=1.1 em, minimum height= 1em, rounded corners= 4pt]

\begin{figure}[h]
\centering
  \begin{tikzpicture}
    \coordinate  (Eq1) at (1,2);
    \coordinate  (Eq2) at (1,1);

    \node[block] at (1,2) (block1) {};
    \node at (Eq1) {$A =  B + C$};
    \node at (Eq2) {$D = E + F$};

    \node[block] at (0, -0.5) (block2) {Blah};
    \draw[->] (block1) -- (block2);

\end{tikzpicture}
\end{figure}
\end{document}

However, it is a huge pain to by hand play around with the node placement of the blocks to highlight the corresponding part of the equation. I'd like a way to specify the node. Attempting to do such, I played around with \tikz{\node} within the equation but to no avail as follows

\documentclass[12pt]{article}

\usepackage{amsmath,amssymb,latexsym}
\usepackage{tikz}


\begin{document}


\tikzstyle{block} = [draw,fill=blue!60,minimum width=1.1 em, minimum height= 1em, rounded corners= 4pt]

\begin{figure}[h]
\centering
  \begin{tikzpicture}
    \coordinate  (Eq1) at (1,2);
    \coordinate  (Eq2) at (1,1);

%\node[block] at (1,2) (block1) {};
    \node at (Eq1) {$A =  \tikz{ \node[fill=blue!60, rounded corners = 4pt, minimum size = 1 em]
        (block1) {$B$};} + C$};
    \node at (Eq2) {$D = E + F$};

    \node[block] at (0, -0.5) (block2) {Blah};
    \draw[->] (block1) -- (block2);

\end{tikzpicture}
\end{figure}
\end{document}

The B is no longer aligned with the other parts of the equation and the arrow is stuck in the second block. Can you suggest a good way to go about doing this? Thanks!

EDIT
Based on the first response (Thanks by the way!) I want to add that at this point I'd like to still do this all within a tikz picture. For my paired down example below I know it make senses to just do \begin\end{equation} but for my actual application I'm hoping to have a lot more going on and just this one equation arbitrarily placed in a bigger diagram. Maybe this can be done still using the usual latex equation commands but it seems more natural to approach having it within a tike picture

Best Answer

The following answer builds on your second attempt, and uses a \tikzmark-like command to achieve the annotation effect.

Code

\documentclass[12pt]{article}

\usepackage{amsmath}
\usepackage{tikz}

\newcommand\mytikzmark[3][]{%
  \tikz[remember picture,baseline=(#2.base)]{\node(#2)[outer sep=0pt,#1]{#3};}%
}

\begin{document}

\tikzset{block/.style={draw,fill=blue!60,minimum width=1.1 em, minimum height= 1em, rounded corners= 4pt}}

\begin{figure}[h]
\centering
  \begin{tikzpicture}[remember picture]
    \draw[help lines](-2,-1)grid(3,3); % shows background

    \coordinate  (Eq1) at (1,2);
    \coordinate  (Eq2) at (1,1);

    \node at (Eq1) {$A = \mytikzmark[block]{block1}{$B$}+ C$};
    \node at (Eq2) {$D = E + F$};

    \node[block] at (0, -0.5) (block2) {Blah};
    \draw[->] (block1) -- (block2);
\end{tikzpicture}
\end{figure}

I can also refer to \verb+(block1)+ outside of the figure like \mytikzmark[block]{block3}{this}.
\tikz[remember picture,overlay]{\draw[->](block3)to[bend right](block1);}
\end{document}

Output

enter image description here

By the way, it's recommended that you use \tikzset instead of the deprecated \tikzstyle.

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