Đáp án:
A'B' nhỏ hơn AB là 2cm nên A'B'=5-2=3cm
$\begin{array}{l}
\Delta ABC \sim \Delta A'B'C'\\
\Rightarrow \dfrac{{AB}}{{A'B'}} = \dfrac{{AC}}{{A'C'}} = \dfrac{{BC}}{{B'C'}}\\
\Rightarrow \dfrac{5}{{5 - 2}} = \dfrac{7}{{A'C'}} = \dfrac{9}{{B'C'}}\\
\Rightarrow \left\{ \begin{array}{l}
A'C' = 7:\dfrac{5}{3} = \dfrac{{21}}{5} = 4,2\left( {cm} \right)\\
B'C' = 9:\dfrac{5}{3} = \dfrac{{27}}{5} = 5,4\left( {cm} \right)
\end{array} \right.
\end{array}$
Vậy A'B'=3cm; A'C' = 4,2cm; B'C'=5,4cm