1/F=1/d+1/f rezulta F=d*f/f+d
stim ca d+f=48 cm=0.48m
β=f/d=3 rezulta f=3d, inlocuim in F=d*f/f+d rezulta
F=d*3d/3d+d =3d²/4d ,
fiindca f+d=48cm=4d rezulta d=48/4, d=12 cm
facem inlocuirile F=3*12²cm²/48cm=432/48=9 cm
f=48cm -12cm= 36 cm
raspuns: F=9cm=0.09m, d=12 cm=0.12m, f=36 cm=0.36m