A rectangular book measures 18 x 9. What is the length of its diagonal?

20.1246118 or 20.12

Step-by-step explanation:

First draw the box with the diagonal, as you can see it is split into 2 equal right triangles.

Now use the pythagorean theorem (a^2+b^2=c^2) to find the length of the diagonal

A^2 + b^2 = c^2

18^2 + 9^2 =

324 + 81 = c^2

405 = c^2

take the sq root

= around 21.12