Đáp án:

\(A'B' = 4cm\)

Giải thích các bước giải:

Ta có: 

\(\begin{array}{l}
\dfrac{{AB}}{{A'B'}} = \dfrac{{OA}}{{OA'}} = \dfrac{{12}}{{OA'}}\\
\dfrac{{AB}}{{A'B'}} = \dfrac{{OI}}{{A'B'}} = \dfrac{{OF'}}{{OA' - OF'}} = \dfrac{8}{{OA' - 8}}\\
 \Rightarrow \dfrac{{12}}{{OA'}} = \dfrac{8}{{OA' - 8}} \Rightarrow OA' = 24cm\\
 \Rightarrow \dfrac{{AB}}{{A'B'}} = \dfrac{{12}}{{24}} = \dfrac{1}{2} \Rightarrow A'B' = 4cm
\end{array}\)