Cho ∆AEF ∽ ∆ABC (hình vẽ) nếu AE = 3cm; EB = 2cm; AF = 4cm, thì FC bằng bao nhiêu?

AB = AE + EB = 2 + 3 = 5(cm)

Vì ∆AEF ∽ ∆ABC nên $\frac{{AE}}{{AB}} = \frac{{AF}}{{AC}}$

Suy ra: AC = $\frac{{AB.AF}}{{AE}} = \frac{{20}}{3}\left( {cm} \right)$

FC = AC – AF = $\frac{{20}}{3} - 4 = \frac{8}{3}\left( {cm} \right)$.