\documentclass[a4paper,12pt]{article}
\usepackage[usenames]{color}

\begin{document}
\pagecolor{black} % Background color

\color{white}     % Text color
\definecolor{Cq}{rgb}{1,.2,.2}
\definecolor{CB}{rgb}{.1,.1,.8}
\definecolor{Cv}{rgb}{.1,.7,.7}
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\center

$\lambda_n={{2 L}\over n} $
 
$ $

$f_n=n {v \over {2L}}$

$ $

$n=1,2,3,\ldots$

$ $

$ $

$\lambda_n={{4 L}\over n} $
 
$ $

$f_n=n {v \over {4L}}$

$ $

$n=1,3,5,\ldots$



\end{document}
