It is proven that the diagonals of a rectangle divide it in two congruent triangles.
How is it so?
Here, ABCD is a rectangle.
AC is diagonal of a rectangle.
In △ABC and △CAD
⇒ AB = DC [ Opposite sides of rectangle ]
⇒ BC = AD [ Opposite sides of rectangle ]
⇒ AC = CA [ Common side ]
∴ △ABC ≅ △CAD [ By SSS congruence property ]