Theorem 5: A rectangle is a parallelogram.
We are given that ABCD is a rectangle. That means that all of the angles are the same size. Since a quadrilateral can be broken up into two triangles, there are a total of 360o in the angles of a quadrilateral. If all of the four angles are the same size, they all have to be 90 o. So we can state our "Given" as
Given: All four angles are 90 o.
To prove: ABCD is a parallelogram.