DIN MN║AB⇒CN/NB=CM/MAadun numitorii la numarator si obt BC/NB=AC/AMde unde NB=BC*AM/AC (1)
din PM║BC avemAP/PB=AM/MC la fel ....proportii derivate
(ap+pb)/pb=(am+mc)/MC
deciAB/PB=AC/MC⇒PB=AB*MC/AC (2)
INLOCUIM IN RELATIA NB/BC+PB/AB....PB (2) SI NB (1)
avem(BC*AM/AC)/BC+(AB*MC/AC)/AB=AM/AC+MC/AC=(AM+MC)/AC=AC/AC=1