Hướng dẫn giải

$\Delta ABC\backsim \Delta A'B'C'$ $\Rightarrow \frac{AB}{A'B'}=\frac{AC}{A'C'}=\frac{BC}{B'C'}=\frac{2}{3}$ (1)

$\Delta A'B'C'\backsim \Delta A''B''C''$ $\Rightarrow \frac{A'B'}{A''B''}=\frac{A'C'}{A''C''}=\frac{B'C'}{B''C''}=\frac{3}{5}$ (2)

Vậy $\Delta A''B''C''\backsim \Delta ABC$ $\Rightarrow \frac{A''B''}{AB}=\frac{A''C''}{AC}=\frac{B''C''}{BC}=\frac{5}{2}$