According to the properties of a rectangle
  1) opposite angles of a rectangle are equal.
  2) the sum of two adjcent angles of  rectangle is equal to 180*
Let's assume...
In rectangle ABCD 
we know that angle A and angle B are two adjacent angles whose sum is equal to 180* ...i.e...A=B=90*
and angle A and angle D are equal acc... to property no..1 and angle B and C are equal too...
Thus,angles A=B=C=D=90 which is  right angle...
I hope diz helped you... if yes ...plzzz...mark as bzt..