Bài 4:
a) A'B' là ảnh thật.
b) Ta có:
\(\begin{array}{l}
\dfrac{{AB}}{{A'B'}} = \dfrac{{OA}}{{OA'}} = 2\\
\dfrac{{AB}}{{A'B'}} = \dfrac{{OI}}{{A'B'}} = \dfrac{{OF'}}{{OA' - OF'}} = \dfrac{{30}}{{OA' - 30}}\\
\Rightarrow 2 = \dfrac{{30}}{{OA' - 30}} \Rightarrow OA' = 45cm\\
\Rightarrow OA = 90cm
\end{array}\)
Bài 5:
\(\begin{array}{l}
\dfrac{{AB}}{{A'B'}} = \dfrac{{OA}}{{OA'}} = 2 \Rightarrow OA = 60cm\\
\dfrac{{AB}}{{A'B'}} = \dfrac{{OI}}{{A'B'}} = \dfrac{{OF'}}{{30 - OF'}} = \dfrac{f}{{30 - f}}\\
\Rightarrow 2 = \dfrac{f}{{30 - f}} \Rightarrow f = 20cm
\end{array}\)
