In math class: Draw a 16 cm long for the base. At each end draw a perpendicular line straight up 8 cm long for the sides. Connect straight across, this should be another 16 cm line.
In building/construction. Lie out a 16 foot board for the base. Attach (nail on) a board to make both ends 8 feet long. Measure the diagonals until each is the same length then nail on the top board another 16 foot board. If the diagonals are equal you will have perpendicular sides.
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