如何更改使用Sweave的beamer演示文稿中的字体？

usepackage{Sweave}

usepackage{helvet}

documentclass[professionalfonts]{beamer}

usepackage[T1]{fontenc}
usepackage{helvet}

usepackage{Sweave}

begin{document}

begin{frame}
    Lorem ipsum dolor sit amet, consectetur adipisicing elit, 
    sed do eiusmod tempor incididunt ut labore et dolore magna
    aliqua. Ut enim ad minim veniam, quis nostrud exercitation
    ullamco laboris nisi ut aliquip ex ea commodo consequat. 
end{frame}

end{document}

usepackage{Sweave}

usepackage{Sweave}