Đáp án:
Bài 5:
a) hình vẽ
b) \(\begin{array}{l}
\dfrac{{AB}}{{A'B'}} = \dfrac{{OA}}{{OA'}} = \dfrac{8}{{OA'}}\\
\dfrac{{AB}}{{A'B'}} = \dfrac{{OI}}{{A'B'}} = \dfrac{{OF'}}{{OA' + OF'}} = \dfrac{{12}}{{OA' + 12}}\\
\Rightarrow \dfrac{8}{{OA'}} = \dfrac{{12}}{{OA' + 12}} \Rightarrow OA' = 24cm \Rightarrow A'B' = 3cm
\end{array}\)
Bài 12:
a) hình vẽ
b)
\(\begin{array}{l}
\dfrac{{AB}}{{A'B'}} = \dfrac{{OA}}{{OA'}} = \dfrac{6}{{OA'}}\\
\dfrac{{AB}}{{A'B'}} = \dfrac{{OI}}{{A'B'}} = \dfrac{{OF'}}{{OA' - OF'}} = \dfrac{4}{{OA' - 4}}\\
\Rightarrow \dfrac{6}{{OA'}} = \dfrac{4}{{OA' - 4}} \Rightarrow OA' = 12cm\\
\Rightarrow \dfrac{{AB}}{{A'B'}} = \dfrac{1}{2} \Rightarrow A'B' = 1cm
\end{array}\)
Bài 13:
\(\begin{array}{l}
\dfrac{{AB}}{{A'B'}} = \dfrac{{OA}}{{OA'}} = \dfrac{6}{{OA'}}\\
\dfrac{{AB}}{{A'B'}} = \dfrac{{OI}}{{A'B'}} = \dfrac{{OF}}{{ - OA' + OF}} = \dfrac{{12}}{{12 - OA'}}\\
\Rightarrow \dfrac{6}{{OA'}} = \dfrac{{12}}{{12 - OA'}} \Rightarrow OA' = 4cm \Rightarrow AB = \dfrac{8}{3}cm
\end{array}\)
