17.1.5 The Golden Ratio
Core ideaMake one cut in a segment so the short piece is to the long piece as the long piece is to the whole, and that special point is the golden cut, with ratio (sqrt5-1)/2.
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.