In △ABE and △CED, ∠BAC=∠BDC (known), ∠AEB=∠CED (equal to vertex angle).

∴△ABE∽△CED (angular angle), that is, the corresponding sides are proportional: AE:DE=BE:CE.

In △AED and △BEC, ∠AED=∠BEC (vertex angles are equal), and the corresponding sides are proportional: AE:DE=BE:CE (authentication).

∴△AED∽△BEC, ∴ △ DACB = ∠ DBC (two triangles are similar and the corresponding angles are equal).