Highlighting equation with arrow

Question

I want to create an equation with highlighting as shown bellow.

But using following code:

\documentclass[10pt,a4paper]{article}  
\usepackage[utf8]{inputenc}  
\usepackage{amsmath}  
\usepackage{amsfonts}  
\usepackage{amssymb}   
\usepackage{graphicx}
\usepackage{tikz}  
\usetikzlibrary{arrows,shapes,trees,positioning}  
\author{Apurba Paul}  
\begin{document}
\newcommand{\tikzmark}[1]{\tikz[baseline,remember picture] \node (#1) {};} 
\tikzset{square arrow/.style={to path={-- ++(-10,-.25) -| (\tikztotarget)}}}  

\begin{gather*}  
  y=\left[\frac{a}{\tikzmark{a}b}\frac{c}{\tikzmark{b}d}\right]\\  
  \tikz[remember picture]{\node(c){1'st part};}\qquad
  \tikz[remember picture]{\node(d){2'nd picture};}
  \tikz{\draw[->](a.south)to (c.north);}
  \tikz{\draw[->](b.south)to (d.north);}   
\end{gather*}  
\end{document}

But I am getting this

Answer 1

Add the options remember picture and overlay to the connecting drawing commands:

\documentclass[10pt,a4paper]{article}  
\usepackage[utf8]{inputenc}  
\usepackage{amsmath}  
\usepackage{amsfonts}  
\usepackage{amssymb}   
\usepackage{graphicx}
\usepackage{tikz}  
\usetikzlibrary{arrows,shapes,trees,positioning}  
\author{Apurba Paul}  
\begin{document}

\newcommand{\tikzmark}[1]{\tikz[baseline,remember picture] \coordinate (#1) {};}

\tikzset{
  square arrow/.style={
    to path={-- ++(-10,-.25) -| (\tikztotarget)}
  }
}

\begin{gather*}  
  y=\left[\frac{a}{\tikzmark{a}b}\frac{c}{\tikzmark{b}d}\right]\\[2ex]
  \tikz[remember picture]{\node(c){1'st part};}\qquad
  \tikz[remember picture]{\node(d){2'nd picture};}
  \tikz[remember picture,overlay]{
    \draw[->] (a.south)++(.25em,-.3ex) to (c.north) ;
    \draw[->] (b.south)++(.25em,-.3ex) to (d.north) ;
  }  
\end{gather*}  
\end{document}

Answer 2

Without tikz:

\documentclass{article}
\usepackage{MnSymbol}
\usepackage{stackengine}
\renewcommand\useanchorwidth{T}
\usepackage{graphicx}
\stackMath
\begin{document}
\[
y = \bigg[
  \def\stackalignment{r}
  \stackunder{%
    \stackunder{\displaystyle\frac{a}{b}}{\scalebox{2}{$\swarrow$}\,}%
    }{\scriptstyle\mathsf{First~Part}~~}
  \def\stackalignment{l}
  \stackunder{%
    \stackunder{\displaystyle\frac{c}{d}}{\,\,\scalebox{2}{$\searrow$}}%
    }{\scriptstyle~~\mathsf{Second~Part}}
\bigg]
\]
\end{document}

And if you wished to do it without the MnSymbol package, you could use \rotatebox to get the tilted arrow:

\documentclass{article}
\usepackage{stackengine}
\renewcommand\useanchorwidth{T}
\usepackage{graphicx}
\stackMath
\begin{document}
\[
y = \bigg[
  \def\stackalignment{r}
  \stackunder{%
    \stackunder{\displaystyle\frac{a}{b}}{%
      \scalebox{1.5}{\rotatebox{-30}{$\downarrow$}}\,}%
    }{\scriptstyle\mathsf{First~Part}~~}
  \def\stackalignment{l}
  \stackunder{%
    \stackunder{\displaystyle\frac{c}{d}}{%
      \,\scalebox{1.5}{\rotatebox{30}{$\downarrow$}}}%
    }{\scriptstyle~~\mathsf{Second~Part}}
\bigg]
\]
\end{document}

Answer 3

Just for fun with PSTricks.

\documentclass[preview,border=12pt,12pt]{standalone}% change it back to your own document class
\usepackage{amsmath}
\usepackage{pst-node}

\begin{document}
$
\displaystyle
\!
\begin{gathered}
y = [\rnode[b]{L}{\frac{a}{b}}\rnode[b]{R}{\frac{c}{d}}]\\[12pt]
\rnode[t]{LT}{\text{First part}}\qquad \rnode[t]{RT}{\text{Second part}}
\psset{nodesep=3pt,arrows=->}
\ncline{L}{LT}\ncline{R}{RT}
\end{gathered}
$
\end{document}

Answer 4

You can create an empty picture and use the overpic package to put the different components on specific places in the picture.

At first: create a white empty picture with arbitrarily size, name it empty.jpg and then put it in the same folder like your tex file.

Below is the MWE. You have just to play arround with the coordinates. But beware: there are two coordinate systems. The one from overpic, and the one from tikz.

\documentclass{article}  
\usepackage{subfigure}    
\usepackage{overpic}  
\usepackage{tikz} 


\begin{document}  

\begin{figure}    
\centering  
\subfigure{  
\begin{overpic}[width=5cm,height=5cm]{empty.jpg}\label{eq:equation1}  
\put(45,50){  $y=\left[\frac{a}{b}\frac{c}{d}\right]$ }  
\put(45,30){  

\begin{tikzpicture}[scale=0.1]  
\draw[->,thin] (0,0) -- (-5,-5);   
\draw[->,thin] (2,0) -- (7,-5);  
\end{tikzpicture}       }  

\put(20,20){1'st part}  
\put(60,20){2'nd picture}  

\end{overpic}           }  
\end{figure}  
\end{document}