Đáp án:

a) Hình vẽ

b) \(OA' = 9cm\) ; \(A'B' = 0,25cm\)

c) \(AB = 1,5cm\)

Giải thích các bước giải:

a) Hình vẽ

b) Ta có:

\(\begin{array}{l}
\dfrac{{AB}}{{A'B'}} = \dfrac{{OA}}{{OA'}} = \dfrac{{18}}{{OA'}}\\
\dfrac{{AB}}{{A'B'}} = \dfrac{{OI}}{{A'B'}} = \dfrac{{OF'}}{{OA' - OF'}} = \dfrac{6}{{OA' - 6}}\\
 \Rightarrow \dfrac{{18}}{{OA'}} = \dfrac{6}{{OA' - 6}} \Rightarrow OA' = 9cm\\
 \Rightarrow A'B' = 0,25cm
\end{array}\)

c) Ta có:

\(\begin{array}{l}
\dfrac{{AB}}{{A'B'}} = \dfrac{{OA}}{{OA'}} = \dfrac{{OA}}{{28}}\\
\dfrac{{AB}}{{A'B'}} = \dfrac{{OI}}{{A'B'}} = \dfrac{{OF'}}{{OA' - OF'}} = \dfrac{{12}}{{28 - 12}} = \dfrac{3}{4}\\
 \Rightarrow \dfrac{{OA}}{{28}} = \dfrac{3}{4} \Rightarrow OA = 21cm\\
 \Rightarrow AB = 1,5cm
\end{array}\)