If in a right-angled triangle a perpendicular is drawn from the right angle to the base, the triangles adjoining the perpendicular are similar both to the whole and to one another.

In equal triangles which have one angle equal to one angle the sides about the angle are reciprocally proportional; and those triangles which have one angle equal to one angle, and in which the sides about the equal angle are reciprocally proportional are equal.

In right angled triangles, the figure on the side subtending the right angle is equal to the similar and similarly described figures on the sides containing the right angle