BB'||DD" (muchii laterale in cub)
[BB']≡[DD'] (muchii laterale in cub)
deci BB'D'D patrulater cu 2 lat opuse || si congruente, BDD'B' paralelogram⇒bd||b'd'.
C.C.T.D.
as simple as that!!
de fapt ,pt ca BB'⊥(ABC)⇒BB'⊥BD⊂(ABC),⇔m∡B'BD=90°, BDD'B' paralelogram cu un unghi drept, BDD'B' dtreptunghi
