### Theory:

The perimeter of a Rectangle: The perimeter is the total distance around the outside, which can be constructed by adding together the length of each side. Let us consider a rectangle of length $$l$$ units and breadth $$b$$ units. Therefore, perimeter of the rectangle $$ABCD$$.
$$Perimeter$$ $$P =$$ $$(AB + BC + CD + DA)$$ units.
$$P =$$ $$( l+ b + l + b )$$ units.
$$P =$$ $$(2l + 2b)$$ units.
$$P =$$ $$2 (l + b)$$ units.
Thus, the length of perimeter $$l =$$ $$P/2 - b$$ unit.
And, the breadth of perimeter $$b =$$ $$P/2 - l$$ unit.
Area of Rectangle: The area of a rectangle is given by multiplying the width times the height. Let us consider a rectangle of length $$l$$ units and breadth $$b$$ units. Therefore, area of the rectangle $$ABCD$$.
$$Area$$ $$A=$$ $$length × breadth$$
$$A =$$ $$l × b$$ square units.
Thus, the length of the rectangle $$l =$$ $$A/b$$ unit.
And, the breadth of the rectangle $$b =$$ $$A/l$$ unit.
Diagonals of Rectangle: A rectangle has two diagonals they are equal in length and intersect in the middle. The diagonal is the square root of (width squared + height squared). $$Diagonal$$ $$d =$$ $$$\sqrt{{l}^{2}+{b}^{2}}$$$
Where $$l$$  is the length of the rectangle.
Where$$b$$ is the breadth of the rectangle.