6. In a right triangle, the sides that form the right angle are the legs; the longest side (opposite the right angle) is the hypotenuse. Some textbooks say that when two right \ triangles have congruent pairs of legs, the right triangles are congruent by the reason LL. In our work, LL is just a spe- cial case of one of the postulates in this section. Which postu- late is that?