In acute-angled triangles triangles
(BAC) the square on the side opposite the acute angle (BC) is
less than the sum of the squares on the sides containing the
acute angle (AB and AC) by twice the rectangle contained by one
of the sides about the acute angle (AC), namely that on which
the perpendicular falls, and the straight line cut off outside
by the perpendicular towards the acute angle (AH). This
conclusion is equivalent to the law of cosines.