17.1.4 The Basic Properties of Proportions
Core ideaThe two sides of the equation balance like a seesaw, so cross-multiplying or combining terms keeps things even and gives ad=bc and its rearrangements.
Module goal. Picking up the parallel lines and congruence from quadrilaterals, we carry the idea of a 'ratio' between numbers over to the lengths of line segments, building the language of proportional segments that lays the foundation for everything about similarity to come.