ABCD is a quadrilateral such that AB = AD and CB = CD. Prove that AC is the perpendicular bisector of BD.
Given: ABCD is quadrilateral
And AB = AD, CB = CD

To prove : AC is  bisector at BD
In ABC and 
ADC
AB = AD (Given)
BC = CD (Given)
AC = AC (Common)
ABC 
 
ADC           (by SSS congruency)
Then by CPCT,
ÐBAC = ÐDAC …(i)
Now in ABO and 
ADO
AB = AD (Given)
AO = AC (common)
BAO = 
DAO (from i)
ABO 
 
DAO          (by SAS congruency)
Then by CPCT,
 
AOB = 
DOA
BOD = 180°
AOB + 
AOD = 180°                     (
BOD = 
AOB + 
AOD)
AOB + 
AOB = 180°                      (
AOB 
 
AOD)
AOB = 90°                                       (
AOB = 
AOD)
Hence, proved.