All of the right triangles are similar.
.. (long side)/(short side) = 20/x = x/5
.. x^2 = 100 . . . . . . . . "cross multiply"
.. x = √100
.. x = 10
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The geometric mean of "a" and "b" is √(ab). x is the geometric mean of 20 and 5, the segments of the hypotenuse of the larger right triangle.
