Daca AB=12cm si DB=9cm => AD=12cm-9cm=3cm.
Aplicam reciproca T. Thales in Δ ABC:
Verificam daca are loc relatia: AD/DB=AE/EC <=> 3/9=2/6 este adevarat
=> DE║BC
=> ∡ADE≡∡ABC - ca unghiuri corespondente formate de dreptele paralele DE si BC intersectate de secanta AB.