Prove that the diagonal of a Rhombus bisect each other at right angles.
ABCD is rhombus
AB = AD
also diagonals bisect each other
DO = OB
AO is common
Triangle AOD congruent to triangle AOB
so angle AOD = angle AOB…c.p.c.t.
but angle AOD + angle AOB = 180…linear pair
so angle AOD = angle AOB = 90