Refer to the image below. There are two images. Page number mentioned on extreme top right corner.
very nice
prove properly.....imagine .....let us take a paralleogram as that DA andCB are diagnols in which they intersects at triange ABO and triangle COD angle COD is equal to angle AOB....angleOCD is equal to angle ABC because they are alternate angle..side CD is equal to side AB .....therefore by ASA postulate triangle COD is congruent to triangle axiom 1 triangle AOD is congruent to triangle BOC...AO=OC and BO=OD since diognals
since diognal are equal and opposite sides are equa...there fore it is rectangle
If the diagonals of a parallelogram are equal then the figure is a rectangle.
proof: the two diagonals of parallelogram forms a rectangle
         these two triangles are congruent by sss  congruency property.
==>by ASP the corresponding angles become 90 degrees
   HENCE the parallelogram forms a rectangle