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
