Fie M un punct oarecare pe muchia (AB) a tetraedrului ABCD.Paralela prin M la BC taie (AC) in N si paralela prin M la BD taie (AD) in P .Aratati ca NP ||(BCD)
E URGENT!!DAU COROANA !!
AM/MB=AN/NC (Thales in tr.ABC) AM/MB=AP/PD (Thales in trABD) din cele 2 relatii⇒AN/NC=AP/PD⇒reciproca Thales⇒NP||CD dar CD⊂(BCD) deci NP||(BCD) as simple as that!!!