The ancient Egyptians used the following methods to get right angles: they divided a rope into 12 segments with equal length with 13 equidistant knots, a craftsman held the 1 knot and 13 knot of the rope at the same time, two assistants held the fourth knot and the eighth knot respectively, and tightened the rope to get a right triangle.

This is the application of Pythagorean theorem.

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