Solution:

Given:

DE∥BC

.

To find: AD.

In ΔADE and ΔABC

∠A = ∠A (Common)

(Corresponding angles as AB||QR with PQ as the transversal)

∠ADE = ∠ABC

(By AA similarity criteria)

⇒ ΔADE ∼ ΔABC

(Corresponding Parts of Similar Triangles are propositional)

AD/AB = AE/ AC