Show that in a quadrilateral ABCD, AB + BC + CD + DA < 2 (BD + AC)
Given : ABCD is a quadrilateral
To prove : AB + BC + CD + DA + < 2 (BD + AC)
Proof: Since we know that the sum of lengths of any two sides in a triangle should be greater than the third side.
In AOB, AB < OA + OB
In BOC, BC < OB + OC
In COD, CD < OC + OD
In AOD, DA < OD + OA
Adding all of the above,
AB + BC + CD + AD < 2OA + 2OB + 2OC + 2OD
AB + BC + CD + AD < 2((AO + OC) + (DO + OB))
AB + BC + CD + AD < 2(AC + BD)
( AC = AO + OC, BD = DO + OB)
Hence proved.