ABCD is a rectangle.

So angle DCB= angle ABC=90 degrees.

ABCD is a rectangle.

So AB=CD

And BC is the male side, equal.

So the right triangle ABCD and DCB are congruent.

So AC=BD

Diagonal lines of parallelograms are equally divided.

So AO=BO

Angle AOB = 60

An isosceles triangle has an angle of 60 degrees.

So it is an equilateral triangle.

2、

Triangle ABO is an equilateral triangle.

AB=4

So AO=BO=4.

So AB=CD=2AO=8cm.

3、

The two diagonals of a rectangle are equal.