Đáp án:
\(\begin{array}{l}
\Delta OAB \sim \Delta OA'B'\\
\Rightarrow \dfrac{{A'B'}}{{AB}} = \dfrac{{OA'}}{{OA}} \Rightarrow \dfrac{{h'}}{h} = \dfrac{{d'}}{d}(dpcm)\\
\Delta OIF'\pi \sim \Delta A'B'F'\\
\Rightarrow \dfrac{{A'B'}}{{OI}} = \dfrac{{A'F'}}{{OF'}} \Rightarrow \dfrac{{A'B'}}{{AB}} = \dfrac{{OF' - OA'}}{{OF'}}\\
\Rightarrow \dfrac{{d'}}{d} = \dfrac{{f - d'}}{f} \Rightarrow \dfrac{1}{f} = \dfrac{1}{{d'}} - \dfrac{1}{d}(dpcm)
\end{array}\)
