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Β x = β70
Step-by-step explanation:
The triangles shown are all similar, so corresponding sides are proportional.
Β hypotenuse/long side = x/7 = (7+3)/x
Cross-multiplying gives ...
Β x^2 = 7(10)
Β x = β70
You will note that the value of x (the long side) is the geometric mean of the long segment and the whole hypotenuse.
A similar "geometric mean" relation holds for the short side:
Β BC = β(CDΒ·CA)
There is yet another "geometric mean" relation that holds for the altitude:
Β BD = β(CDΒ·AD)
You may see these again.