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 360^{o} 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.