In △ABC, points D and E are taken on sides AB and AC respectively such that ∠ADE=∠ABC and ∠AED=∠ACB.
(i) Show that DE∥BC.
(ii) Prove that △ADE∼△ABC.
(iii) Hence, obtain the relation BCDE=ABAD.
(iv) If AD=3 cm, AB=5 cm and BC=7 cm, find the length of DE.