http://mathworld.wolfram.com/Pyramid.html
Pay attention to use of the words 'therefore' and 'only possible'. Also the simple calculation for the lateral surface area. Most of this knowledge isnt even required if you adopt a systematic approach from the base size you desire and just keep on piling on slightly smaller bases, like the steppe pyramid does, or the ziggurats of the incas. Receeding tiers to learn the math, then start making sloped stones once you've got it all down.
