DAPPH
as dyslexik as I'm daft
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All these calculations seem a bit complex. for a rectangle, peg it out by eye, then a piece of string from one corner to the one diagonally opposite, then the string across the other diagonal. If it isn't the same length, move the corner pegs. Bet that is how the Egyptians did it.
We were told it's likely they use very thin ropes made of plaited up human hair and stretched them with a known weight over the cross pieces to get them equally tensioned and not drooping & used the 3, 4, 5 triangulation method.
If the four sided figure is not square , one side being shorter than but parallel & central to the opposite side the diagonals can still be the same lengths but the corners won't be square
Egyptian Mathematics - The Story of Mathematics
storyofmathematics.com/egyptian.html
They were also aware, long before Pythagoras, of the rule that a triangle with sides 3, 4 and 5 units yields a perfect right angle, and Egyptian builders used ropes knotted at intervals of 3, 4 and 5 units in order to ensure exact right angles for their stonework (in fact, the 3-4-5 right triangle is often called "Egyptian").