Đáp án:
Ta có:
\(\begin{array}{l}
\dfrac{{AB}}{{A'B'}} = \dfrac{{OA}}{{OA'}} = \dfrac{{15}}{{OA'}}\\
\dfrac{{AB}}{{A'B'}} = \dfrac{{OI}}{{A'B'}} = \dfrac{{OF'}}{{OA' - OF'}} = \dfrac{5}{{OA' - 5}}\\
\Rightarrow \dfrac{{15}}{{OA'}} = \dfrac{5}{{OA' - 5}} \Rightarrow OA' = 7,5cm\\
\Rightarrow \dfrac{{AB}}{{A'B'}} = 2 \Rightarrow A'B' = 6cm
\end{array}\)