skeeter said:for a rectangular prism with dimensions l, w, and h, the diagonal length is
\(\displaystyle \sqrt{l^2 + w^2 + h^2}\)
Loren said:I have a tough time NOT responding to this question. It seems to me that the questioner is content to use the given formula without understanding why it works. I would encourage him/her to draw a diagonal on the side face of the cube and calculate it's measure --- possibly \(\displaystyle \sqrt{k^2+1^2}\). Then he/she could work with the inner right triangle where AB is the hypotenuse. The result would be \(\displaystyle (AB)^2=(\sqrt{k^2+1^2} )^2 + 1^2\). From there he/she could arrive at the final result.