Find the measure of one interior angle of a regular octagon. If the diagonals of a quadrilateral are congruent, then the quadrilateral is a rectangle. False - Might be an isosceles trapezoid or another quadrilateral unless the figure was given to be a parallelogram too. All squares are rectangles. A rectangle is a quadrilateral with 4 right angles.
Squares fit that definition.
They just have extra properties. If a quadrilateral has 4 congruent sides then it is a square. To be a square you need extra properties such as 4 right angles or congruent diagonals. If both pair of opposite angles of a quadrilateral are congruent then it is the quadrilateral is a parallelogram. One of the methods of proving a quadrilateral is a parallelogram is to show both pair of opposite angles congruent.