So it is not the maximum value. Calculate the diagonal of a rectangle in which its height be 20 cm and width be 25 cm. Simply plug the length and the diagonal into the formula above.
You can calculate only the upper excluded limit of value of area of a rectangle say A m2 when merely its diagonal length is known as say D m using the formula.
The area of a rectangle depends on its sides. A is the length of the rectangle and b is the width of the rectangle. What does it mean to find the diagonal of a rectangle. Remember the diagonal is always longer than the length of the rectangle.