Question

May someone explain how to find the diagonal of a rectangular solid ?

Answer 1

L = length
W = width
H = height
D = diagonal of the 3D rectangular solid
D = sqrt(L^2+W^2+H^2)
which is essentially a 3D extension of the 2D pythagorean theorem c = sqrt(a^2+b^2)

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For example, if 
L = 3, W = 4 and H = 10,
then
D = sqrt(L^2+W^2+H^2)
D = sqrt(3^2+4^2+10^2)
D = sqrt(9+16+100)
D = sqrt(125)
D = 11.1803 which is approximate

In this example, the length of the diagonal is roughly 11.1803 units long
This is the distance from one corner to the furthest opposite corner. The diagonal goes through the solid itself and does not simply stay on one face only.