∵AB<OA+OB,BC<OB+OC,CD<OC+OD,DA<OD+OA
∴AB+BC+CD+DA<2(OA+OB+OC+OD)
∴1/2(AB+BC+CD+DA)<AC+BD
又AC<DA+CD,BD<CD+BC,AC<AB+BC,BD<AB+DA
∴2(AC+BD)<2(AB+BC+CD+DA)
∴AC+BD<AB+BC+CD+DA
从而得到:1/2(AB+BC+CD+DA)<AC+BD<AB+BC+CD+DA
∵AB<OA+OB,BC<OB+OC,CD<OC+OD,DA<OD+OA
∴AB+BC+CD+DA<2(OA+OB+OC+OD)
∴1/2(AB+BC+CD+DA)<AC+BD
又AC<DA+CD,BD<CD+BC,AC<AB+BC,BD<AB+DA
∴2(AC+BD)<2(AB+BC+CD+DA)
∴AC+BD<AB+BC+CD+DA
从而得到:1/2(AB+BC+CD+DA)<AC+BD<AB+BC+CD+DA