Question

prove that diagonals of square are equal and bisect each other at 90degree

Answer 1

make a square ABCD where ab=bc=cd=da and diagonals AC and BD.
in ACD and BDC
AD=BC(given)
angle ADC=angle BCD(all angles of square are 90)
DC=DC(common)
ACD congruent to BDC
AC=BD(cpct)