Ta có OA = d = 120cm, OF = OF'= 8cm, AB = h = 40cm

$\Delta$BOA   $\Delta$B'OA' , $\Delta$KOF'   $\Delta$B'A'F'

=> $\frac{OA}{OA'}=\frac{BA}{B'A'}$ và $\frac{OK}{A'B'}=\frac{OF'}{A'F'}$

$\Leftrightarrow$ $\frac{120}{OA'}=\frac{40}{A'B'}$ và $\frac{40}{A'B'}=\frac{8}{A'F'}$ ( OK = AB = 40cm)

$\Leftrightarrow$ OA' = 15.A'F'

Mà OF' + A'F' = OA' => OA' = $\frac{15}{14}$. OF' =  $\frac{15}{14}$.8 = $\frac{60}{7}$ (cm)

=> A'B' = $\frac{OA'}{120}.AB$ = $\frac{60.40}{120.7}$ = 2,86 (cm)

Vậy ảnh cao 2,86cm