17.1.3 Proportional Segments
Core ideaWhen four sticks line up so their lengths pair perfectly inside and out, we write a:b=c:d, which means ad=bc.
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.