Question: I want to include latex packages in R markdown pdf and beamer. 

Could you help me in including usepackage command in rmarkdown beamer file?

Thanks a lot.

Answer: 
A simple solution would be to have your header setup like this:

```
---
title: "Document title"
author: "Author's name"
date: "Document date"
output: beamer_presentation
header-includes:
- \usepackage[brazil]{babel}
- \usepackage{graphicx}
- \usepackage[a4paper]{geometry}
---

```
The YAML header of the Rmd would look like:

```
---
title: "Habits"
output:
  pdf_document:
    includes:
      in_header: header.tex
---

```

Then, in the file header.tex, include any usepackage statements you need, eg. `\usepackage{pdflscape}`

See <http://rmarkdown.rstudio.com/pdf_document_format.html> for more information.