What is an equilateral rectangle?
It's a square. A rectangle is any polygon with two pairs pf
parallel sides all intersecting at perpendicular angles. It's
diagonals are also congruent. This is also true with a square. So,
a square could also be considered a rectangle. The only difference
is that a square's sides all have to be the same length, and a
rectangle's doesn't. 'Equilateral' means all the sides are equal.
Therefore, an equilateral rectangle is a square.