Bài làm

Ta có AB = 2 A’B’

OA+OA’= 6 cm

Ta có $\Delta ABO\backsim \Delta A'B'O\,(g.g)$

$\Rightarrow \frac{AB}{A'B'}=\frac{OA}{OA'}\Leftrightarrow \frac{2}{1}=\frac{OA}{OA'}\Rightarrow OA=4cm;OA'=2cm$

Ta có $\Delta IOF'\backsim \Delta B'A'F'\,(g.g)$ $\Rightarrow \frac{IO}{B'A'}=\frac{\text{OF }\!\!'\!\!\text{ }}{A'F'}$

Mà AB = IO  nên $\Rightarrow \frac{AB}{B'A'}=\frac{\text{OF }\!\!'\!\!\text{ }}{A'F'}\Leftrightarrow \frac{2}{1}=\frac{f}{OA'-F}\Leftrightarrow \frac{2}{1}=\frac{f}{2-f}\Rightarrow f=1,33cm$

Đáp án A