Bài 1:
a) Hình vẽ
b) Ta có:
\(\begin{array}{l}
\dfrac{{AB}}{{A'B'}} = \dfrac{{OA}}{{OA'}} = \dfrac{{50}}{{20}} = 2,5 \Rightarrow A'B' = 2cm\\
\dfrac{{AB}}{{A'B'}} = \dfrac{{OJ}}{{A'B'}} = \dfrac{{OF'}}{{OA' - OF'}} = \dfrac{f}{{20 - f}}\\
\Rightarrow 2,5 = \dfrac{f}{{20 - f}} \Rightarrow f = \dfrac{{100}}{7}cm
\end{array}\)
Bài 2:
a) Ta có:
\(\dfrac{{AB}}{{A'B'}} = \dfrac{{OA}}{{OA'}} \Rightarrow 3 = \dfrac{{OA}}{{10}} \Rightarrow OA = 30cm\)
b) Ta có:
\(\begin{array}{l}
\dfrac{{AB}}{{A'B'}} = \dfrac{{OI}}{{A'B'}} = \dfrac{{OF'}}{{OF' - OA'}} = \dfrac{f}{{f - 10}}\\
\Rightarrow 3 = \dfrac{f}{{f - 10}} \Rightarrow f = 15cm
\end{array}\)
