Question

What is the diagonal of a rectangle with length 48 inches and width 40 inches?

Answer 1

Use the Pythagorean Theorem: $a^2 + b^2 = c^2$
You can do this because when you cut the rectangle in half with the diagonal, it forms a right triangle, making the diagonal the hypotenuse (c). 

$40^2 + 48^2 = c^2$

$1600 + 2304 = c^2$

$3904 = c^2$

$\sqrt{3904} = \sqrt{c^2}$

$62.48 = c$

Diagonal: 62.48 inches

Answer 2

I think its D=w2+l2=402+482≈ 62.482in