Answer
$\sqrt {r^{2}+ s^{2}}$
Work Step by Step
The length $(r)$ and width $(s)$ of the rectangle forms a right-angled triangle with the diagonal of the rectangle.
In this triangle, the diagonal forms the hypotenuse.
From the Pythagoras theorem, we know that
$\sqrt {a^{2}+b^{2}}$ $=$ $c$
Substituting r and s into the formula, we get
$\sqrt {r^{2}+ s^{2}}$ $=$ $D$
Where,
$r$ $=$ Length of rectangle
$s$ $=$ Breadth of rectangle
$D$ $=$ Diagonal
