Đáp án:
\(f = - \dfrac{{50}}{9}cm\)
Giải thích các bước giải:
Ta có:
\(\dfrac{{{d_2}'}}{{{d_1}'}} = 2 \Rightarrow {d_2}' = 2{d_1}' \Rightarrow \left\{ \begin{array}{l}
{d_1} = - 5cm\\
{d_2} = - 10cm
\end{array} \right.\)
Mà:
\(\begin{array}{l}
\dfrac{1}{f} = \dfrac{1}{{{d_1}}} + \dfrac{1}{{{d_1}'}} = \dfrac{1}{{{d_1}}} - \dfrac{1}{5}\\
\dfrac{1}{f} = \dfrac{1}{{{d_2}}} + \dfrac{1}{{{d_2}'}} = \dfrac{1}{{{d_1} - 40}} - \dfrac{1}{{10}}\\
\Rightarrow \dfrac{1}{{{d_1}}} - \dfrac{1}{5} = \dfrac{1}{{{d_1} - 40}} - \dfrac{1}{{10}} \Rightarrow {d_1} = 50cm\\
\Rightarrow f = - \dfrac{{50}}{9}cm
\end{array}\)