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4. Add two given squares.
Make a right angle, and on its sides place the sides of the squares. These lengths will be the small sides of your right angled triangle; then draw the longest side, and you have the side of a square equal to the other two.
5. Double a square. (fig. 3.)
The triangle must be a rectangular isoceles, and the two small squares will then be equal to each other, or united they will be equal to the large square.