multi-line equation in RMarkdown

Question

I am trying to create a multi-line equation in RMarkdown. I have tried the following code, extracted from http://www.rstudio.com/ide/docs/authoring/using_markdown_equations,

$$
\begin{aligned}
  \dot{x} & = \sigma(y-x) \\
  \dot{y} & = \rho x- y - xz 
\end{aligned}
$$

however the knitred HTML document gives a single-line equation:

Answer 1

This code produced 2 HTML lines, as desired, when cut and paste into RStudio in Windows 7 with knitr. If the second slash at the end of the 3rd line was removed, it produced the result shown in example.