ΔABC dreptunghic in A
mas<B=60°=> mas<C=30° => (T.30.60.90) AB=BC/2.
daca E mijloc BC => DE=CE=BC/2.
in ΔABE cunoastem BE=AB=(BC/2) => ΔABE isoscel.
dar in ΔABE exista mas<B=60° => ΔABE echilateral( conform definitiei)
mas<DAE=mas<A-mas<DAB
mas<DAE=90-60
mas<DAE=30°
