Answer:
The first way to prove that the diagonals of a rectangle are congruent is to show that triangle ABC is congruent to triangle DCB
Here is what is given: Rectangle ABCD
Here is what you need to prove: segment AC ≅ segment BD
Since ABCD is a rectangle, it is also a parallelogram.
Since ABCD is a parallelogram, segment AB ≅ segment DC because opposite sides of a parallelogram are congruent.