What is the formula for finding the diagonal of a cube and a cuboid?

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https://www.meritnation.com/ask-answer/question/what-is-the-formula-for-the-longest-diagonal-of-a-cube/surface-areas-and-volumes/6648298
https://www.meritnation.com/ask-answer/question/how-to-derrive-the-equation-for-finding-the-diagonal-of-a-cu/surface-areas-and-volumes/6473239