\documentclass{article}
\usepackage{tikz,amsmath}
\usepackage[paperwidth=21cm,paperheight=21cm,margin=0cm]{geometry}
\setlength{\parindent}{0pt}
\usepackage{tikz}
\newcommand{\rep}{%
\clip (-1,-1) rectangle (20,20);
\draw[gray,dashed] (-1,-1) grid (20,20);
\draw[->,>=latex] (-1,0) -- (20,0);
\draw[->,>=latex] (0,-1) -- (0,20);
\draw[thick] plot[domain=-1:20,samples=100] (\x,{\x*\x*\x/60-\x*\x/3+20});
\fill[blue] (5,{55/4}) circle (4pt) node[above right,scale=2] {$A$};
\node[gray,above left,scale=2] at (20,-1) {mathweb.fr};
}
\begin{document}
\foreach\a in {19,18.75,18.5,...,5.25}
{
\begin{tikzpicture}
\rep
\fill[orange] (\a,{\a*\a*\a/60-\a*\a/3+20}) circle (4pt) node[above right,scale=2] {$M$};
\draw[very thick,orange] plot[domain=-1:20] (\x,{(\a*\a*\a/60-\a*\a/3+20-55/4)*\x/(\a-5)+55/4-5*(\a*\a*\a/60-\a*\a/3+20-55/4)/(\a-5)});
\end{tikzpicture}
}
\begin{tikzpicture}
\rep
\fill[orange] (5,{55/4}) circle (2pt) node[below left,scale=2] {$M$};
\draw[very thick,orange] plot[domain=-1:20] (\x,{-25*(\x-5)/12+55/4});
\end{tikzpicture}

\end{document}