## Precalculus (6th Edition)

$\sqrt {r^{2}+ s^{2}}$
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