Each leg of the triangle is the mean proportional of the hypotenuse and the segment of the hypotenuse that is adjacent to the leg:
$$a^{2}=fc \;\textrm{ja}\;b^{2}=gc$$
See also:
Euclidean Geometry Theorem FAQ
What does the theorem state?
Each leg of a right triangle is the mean proportional between the hypotenuse and the adjacent segment.
What is it used for?
It connects the side lengths of a right triangle with the altitude to the hypotenuse.
