Răspuns:
Explicație pas cu pas:
CE=3·DE, dar CD=AB=12cm, CE+DE=12, ⇒3·DE+DE=12, 4·DE=12, ⇒DE=3cm. Atunci CE=CD-DE=12-3=9cm.
CD║AB, deci ΔFCE≅ΔFBA, ⇒FC/FB=CE/BA, dar FB=FC+CB=FC+6
Atunci FC/(FC+6)=9/12=3/4, ⇒4·FC=3·(FC+6), ⇒ 4·FC=3·FC+18, ⇒4·FC-3·FC=18, ⇒FC=18cm.
