Bài 4:
a) Hình vẽ
b) Ta có:
\(\begin{array}{l}
\dfrac{{AB}}{{A'B'}} = \dfrac{{OA}}{{OA'}} = \dfrac{1}{3}\\
\dfrac{{AB}}{{A'B'}} = \dfrac{{OI}}{{A'B'}} = \dfrac{{OF'}}{{OA' - OF'}} = \dfrac{6}{{OA' - 6}}\\
\Rightarrow \dfrac{1}{3} = \dfrac{6}{{OA' - 6}} \Rightarrow OA' = 24cm
\end{array}\)
\( \Rightarrow OA = 8cm\)
Bài 3:
a) Hình vẽ
b) Ta có:
\(\begin{array}{l}
\dfrac{{AB}}{{A'B'}} = \dfrac{{OA}}{{OA'}} = \dfrac{9}{{OA'}}\\
\dfrac{{AB}}{{A'B'}} = \dfrac{{OI}}{{A'B'}} = \dfrac{{OF'}}{{OA' - OF'}} = \dfrac{6}{{OA' - 6}}\\
\Rightarrow \dfrac{9}{{OA'}} = \dfrac{6}{{OA' - 6}} \Rightarrow OA' = 18cm\\
\Rightarrow A'B' = 6cm
\end{array}\)