3 m= 300cm\(\Delta ABO\infty\Delta A'B'O\Rightarrow\dfrac{AB}{A'B'}=\dfrac{OA}{OA'}\left(1\right)\)
\(\Delta OIF'\infty\Delta A'B'F'\Rightarrow\dfrac{OF'}{A'F'}=\dfrac{OI}{A'B'}\)
\(\Rightarrow\dfrac{OF'}{OA'-OF'}=\dfrac{AB}{A'B'}\left(2\right)\)
(1,2) \(\Rightarrow\dfrac{OF'}{OA'-OF'}=\dfrac{OA}{OA'}\Rightarrow OA'\approx5,1\)