Cannot show the matrix correctly

[ghost]

When inputting a matrix, I cannot get a newline by using \\ For example,

\begin{matrix}
a & b \\
c & d 
\end{matrix}

cannot show the matrix correct. But using a "\newline"

\begin{matrix}
a & b \newline
c & d 
\end{matrix}

it wokrs.

[ghost]

It may because the first backslash is used as an escape character, so it cannot be identified in HTML.