Given: DE || BC
DE = 4cm, BC = 8cm and area (ΔADE) =25 sq. cm
In ΔABC and ΔADE
∠B = ∠D [∵ DE || BC and AB is transversal, Corresponding angles are equal]
∠C = ∠E [∵ DE || BC and AC is transversal, Corresponding angles are equal]
∠BAC =∠DAE [common angle]
∴ ΔABC ~ΔADE [by AAA similarity]
Now, we know that, the ratio of two similar triangles is equal to the square of the ratio of their corresponding sides.