
\begin{tikzpicture}

\node [anchor=west] (eq1) at (0,0) {$\sum\limits_{a_1=0}^{l} ... \sum\limits_{a_m=0}^{l} \prod\limits_{j=1}^{m} f(s_j|t_{a_j})$};
\node [anchor=west] (eq1part2) at (eq1.east) {$=$};
\node [anchor=west,inner sep=2pt] (eq1part3) at (eq1part2.east) {$\prod\limits_{j=1}^{m} \sum\limits_{i=0}^{l} f(s_j|t_i)$};
{
\node [anchor=west,inner sep=2pt,fill=red!20] (eq1part3) at (eq1part2.east) {$\prod\limits_{j=1}^{m} \sum\limits_{i=0}^{l} f(s_j|t_i)$};
}



{
\node [anchor=west] (eq2) at ([xshift=5em,yshift=-4.5em]eq1.west) {$\textrm{P}(s|t) = \frac{\epsilon}{(l+1)^{m}} $};
\node [anchor=west,inner sep=2pt] (eq2part2) at ([xshift=-0.3em]eq2.east) {$\prod\limits_{j=1}^{m} \sum\limits_{i=0}^{l} f(s_j|t_i)$};
\node [anchor=east] (eq2label) at ([xshift=-0em,yshift=0.2em]eq2.west) {\small{IBM模型1:}};

\node [anchor=west,inner sep=2pt,fill=red!20] (eq2part2) at ([xshift=-0.3em]eq2.east) {$\prod\limits_{j=1}^{m} \sum\limits_{i=0}^{l} f(s_j|t_i)$};
}

{
\node [anchor=west] (eq3) at ([xshift=5em,yshift=-7.5em]eq1.west) {$\textrm{P}(s|t) = \epsilon$};
\node [anchor=west,inner sep=2pt] (eq3part2) at ([xshift=-0.3em]eq3.east) {$\prod\limits_{j=1}^{m} \sum\limits_{i=0}^{l} a(i|j,m,l) f(s_j|t_i)$};
\node [anchor=east] (eq3label) at ([xshift=-0em,yshift=0.2em]eq3.west) {\small{类似的，IBM模型2:}};
}

\begin{pgfonlayer}{background}
{
\node[rectangle,draw=red,inner sep=2pt] [fit = (eq2) (eq2part2) (eq2label) (eq3) (eq3part2) (eq3label)] {};
}
\end{pgfonlayer}

{
\draw [->,thick] ([xshift=-1em]eq1part3.south) .. controls +(south:1.3em) and +(north:1.3em) .. ([xshift=1em]eq2part2.north);
}

\end{tikzpicture}



