b)
Tóm tắt:
AB = 2cm
OF = OF' = 8cm
OA = 24cm
A'B' = ?
OA' = ?
Giải:
\(\Delta ABF\sim OIF\)
\(\Rightarrow\dfrac{AB}{OI}=\dfrac{AF}{OF}\Leftrightarrow\dfrac{AB}{A'B'}=\dfrac{OA-OF}{OF}\Leftrightarrow\dfrac{2}{A'B'}=\dfrac{24-8}{8}\)
=> A'B' = 1cm
\(\Delta OAB\sim\Delta OA'B'\)
\(\Rightarrow\dfrac{OA}{OA'}=\dfrac{AB}{A'B'}\Leftrightarrow\dfrac{24}{OA'}=\dfrac{2}{1}\Rightarrow OA'=12cm\)