91. (8th grade) Find all rectangles that can be cut into 13 equal squares.
91. Since all squares are equal, the sides of the rectangle will be divided into equal parts: one into $x$ parts, the other into $y$ parts. There are a total of 13 squares. Therefore, $x y=13$. Since 13 is a prime number, either $x=1$ or $y=1$, then $y=13$ or $x=13$. A rectangle with sides 1 and 13 and similar rectangles can be divided into 13 equal squares.