The Pythagorean Theorem states that for any triangle with one angle at 90 degrees, the area of the squares of the
two smaller sides adds up to be equal to the area of the square of the largest side.
Figure 2 shows how the four triangles can be moved to prove that: c^{2} = a^{2} + b^{2}

In construction, it is common to use the Pythagorean Theorem to check that an angle is 90 degrees.
For example, mark off 4 units in the vertical, 3 units in the horizontal and then check that the
third side is 5 units. If so, the angle is 90 degrees.
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