Đáp án:
\(\dfrac{{300}}{7}cm\) ; \(\infty \) ; \(150cm\) ; \( - 37,5cm\)
Giải thích các bước giải:
* Khi d = 60cm:
\(\dfrac{1}{f} = \dfrac{1}{d} + \dfrac{1}{{d'}} \Rightarrow \dfrac{1}{{25}} = \dfrac{1}{{60}} + \dfrac{1}{{d'}} \Rightarrow d' = \dfrac{{300}}{7}cm\)
* Khi d = 25cm:
\(\dfrac{1}{f} = \dfrac{1}{d} + \dfrac{1}{{d'}} \Rightarrow \dfrac{1}{{25}} = \dfrac{1}{{25}} + \dfrac{1}{{d'}} \Rightarrow d' = \infty \)
* Khi d = 30cm:
\(\dfrac{1}{f} = \dfrac{1}{d} + \dfrac{1}{{d'}} \Rightarrow \dfrac{1}{{25}} = \dfrac{1}{{30}} + \dfrac{1}{{d'}} \Rightarrow d' = 150cm\)
* Khi d = 15cm:
\(\dfrac{1}{f} = \dfrac{1}{d} + \dfrac{1}{{d'}} \Rightarrow \dfrac{1}{{25}} = \dfrac{1}{{15}} + \dfrac{1}{{d'}} \Rightarrow d' = - 37,5cm\)
