How to write matrix or tidy table in Rmarkdown?

I believe most of you are quite familiar using R markdown to write either your analysis report or work sample, some of which requires a lot of formula and table writing. So here is what I find to be useful to do these jobs.

1. Write math matrix

using

mathbf{X} = \left[\begin{array}
{rrr}
1 & 2 & 3 \\
4 & 5 & 6 \\
7 & 8 & 9
\end{array}\right]

2. Create tidy tables using Markdown Table Generator https://www.tablesgenerator.com/markdown_tables
   
   Just type in the values and do copy/paste to your R markdown file.

Hope everyone learns a new skill everyday!