If a pair of opposite sides of a cyclic quadrilateral are equal, prove that its diagonals are also equal.
Given: A pair of opposite sides of a cyclic quadrilateral are equal.
Let ABCD be a cyclic quadrilateral with AD = BC
To Prove: Diagonals are equal, i.e., AC = BD
Proof:
We know that angles is same segment are equal.
Consider segment CD, DAC = DBC
So we can write, DAO = CBO …(i)
Consider segment AB, ADB = ACD
So we can write, ADO = BCO…(ii)
In AOD & BOC,
DAO = CBO (from (i))
ADO = BCO (from (ii))
AOD = BOC (vertically opposite angles)
AOD BOC [AAA congruence rule]
AO = BO (CPCT)…(iii)
DO = CO (CPCT)…(iv)
Adding (iii) and (iv),
AO + OC = BO + OD
Hence, AC = BD
Hence proved