Răspuns:
Explicație pas cu pas:
BD²=AB²+AD²=15²+20²=225+400 =625
BD=√625 =25 cm
AE =C*C/ipot. =15*20/25 =12 cm
ΔADE = drept.
DE²=AD²-AE²=20²-12²=400-144=256
DE=√256 =16 cm
EF⊥CD ⇒FR║BC
DE/DB=EF/BC
16/25 =EF/20
EF=16*20/25 =12 cm
