MN linmie mijlocie in tr ABC, MN||BC, MN=BC/2
PQ linie mijlocie in tr BDC; PQ ||BC. PQ=BC/2
2 laturi opuse paralele si congruente , MNQP paralelogram, cerinta
cazuri a) A si D in semiplane diferite fata de BC
b) Asi D in acel;asi semiplan
c) daca A si D∈mediatoarei BC, MNQP, dreptunghi
d) daca ABC si BCD triunghuri dreptunghice isoscele, MNQP, patrat
e) daca A si D coincid, paralelogramul MNQP degenereaza in segmentul MN, identic NQ