can't display equation in equation mode

Question

I wrote equations and don't understand why some equations are displayed and others not. For example this equation can't be displayed

\begin{quote}
\begin{equation}
Sim_{Dice}=\frac{2\times a}{c_{1}+c_{2}}

\end{equation}
\end{quote}

and this equation is correctly displayed

\begin{quote}
\begin{equation}
I(X,Y)=\sum\limits_{x,y}P(x,y)\ log \ \frac{P(x,y)}{P(x)P(y)}
\end{equation}
\end{quote}

they are in the same document.