13. 如图，$AD⊥ BC$于点$D$，$△ ABD≌△ CFD$，$CF$的延长线交$AB$于点$E$.
(1) 请说明：$CE⊥ AB$；
(2) 若$BC = 7$，$AD = 5$，求$AF$的长.

13. (1) $ ∵ △ABD ≌ △CFD $，$ ∴ ∠BAD = ∠DCF $。$ ∵ AD ⊥ BC $，$ ∴ ∠ADB = 90° $。$ ∴ ∠B + ∠BAD = 90° $。$ ∴ ∠B + ∠BCE = 90° $。$ ∴ ∠CEB = 90° $。$ ∴ CE ⊥ AB $ (2) $ ∵ △ABD ≌ △CFD $，$ ∴ AD = CD = 5 $，$ BD = DF $。$ ∵ BC = 7 $，$ ∴ DF = BD = 7 - 5 = 2 $。$ ∴ AF = AD - DF = 5 - 2 = 3 $