а) $$AC = \frac{1}{4} \cdot AB = \frac{1}{4} \cdot 8 \text{ см} = 2 \text{ см}$$.
б) $$AD = \frac{7}{8} \cdot AB = \frac{7}{8} \cdot 8 \text{ см} = 7 \text{ см}$$.
в) $$DE = \frac{2}{7} \cdot AD = \frac{2}{7} \cdot 7 \text{ см} = 2 \text{ см}$$.
\documentclass[12pt]{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\draw (0,0) -- (8,0);
ode[below] at (0,0) {A};
ode[below] at (2,0) {C};
ode[below] at (4,0) {E};
ode[below] at (7,0) {D};
ode[below] at (8,0) {B};
\end{tikzpicture}
\end{document}
Ответ: AC = 2 см, AD = 7 см, DE = 2 см.