Given: DE ∥ BC

DE = 5 cm

BC = 8 cm

AD = 3.5 cm

To find: AB

∵ DE ∥ BC

∴ By Basic proportionality theorem, we have

………..(i)

Now, in ∆ ADE and ∆ ABC, we have

[By (i)]

∠DAE = ∠BAC [Common angle]

∴ By SAS criterion,

∆ ADE ∼ ∆ ABC