in triunghiul ADC AD<AC<AD+DC
in triunghiul BCD CD <BD<BC+CD
in triunghiul ABC BC < AC<AB+BC
in triunghiul BAD AB <BD<AB+AD
adunam cele 4 inecuatii si obtinem
AB+BC+CD+AD < 2AC+2BD< 2AB+2BC+2CD+2AD
AB+BC+CD+AD< 2 (AC+BD) <2(AB+BC+CD+AD) impartim totul la 2
(AB+BC+CD+AD) :2 <AC+BD< AB+BC+CD+AD
p < AC+BD < 2p
