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.