Refer to the figure: DE is parallel to BC.
ADDB = ?
Consider△ADE and △ABC
∠DAE=∠BAC [Common angle]
∠ADE=∠ABC [right angle]
Therefore by AA similarity criterion, the triangles△ADE and △ABC are similar.
So, ADAB = AEAC
⟹ ADAD+DB = AEAE+EC
⟹ AD+DBAD = AE+ECAE
⟹ 1 + DBAD = 1 + ECAE
⟹ DBAD = ECAE
⟹ ADDB = AEEC