Исходники иллюстрации

\documentclass[tikz]{standalone}\input{pre.tex}\begin{document}\begin{tikzpicture}

        \draw[->] (0,-0.2) -- (0,6.5) node[above]{$T(l)$};
        \draw[->] (-0.2,0) -- (10,0) node[right]{$l$};
        \draw[variable=\l, cyan,line width=1pt] plot[domain=0.03:6.5, samples=300, ] (1.5*\l,{sqrt((1+\l*\l)/\l)});

        \fill[magenta] (0.246,2.5) coordinate (1) circle (2pt);
        \fill[magenta] (9.128,2.5) coordinate (2) circle (2pt);

        \draw[axis, very thin] (1) -- ++(0,-2.5) coordinate (l1);
        \draw[axis, very thin] (2) -- ++(0,-2.5) coordinate (l2);

        \fill[magenta] (0.246,2.5) circle (2pt);
        \fill[magenta] (9.128,2.5) circle (2pt);    

        \draw ($(l1)+(0,5pt)$) -- ($(l1)-(0,5pt)$) node[below]{$l_1$};
        \draw ($(l2)+(0,5pt)$) -- ($(l2)-(0,5pt)$) node[below]{$l_1$};

\end{tikzpicture}\end{document}