Draw in the diagonal of the base, which is the line freom G to F. If the side length of this cube is s, then the length of this diagonal is
d = √(s² + s²) = (√2)s.
Now draw in the diagonal of the cube: draw a line segment from G to B. We have already found that the length of the diagonal GF is d = s√2. Apply the Pythagorean Theorem to the triangle whose sides are s√2 and s:
diagonal of cube = square root of the sum of the squares of s√2 and s:
