28.7.1 Tests for Perpendicular Lines and a Line Perpendicular to a Plane
Core ideaIf a flagpole is perpendicular to two intersecting lines on the ground at once, it's perpendicular to the whole ground.
Module goal. Master the tests and properties of perpendicularity for line-to-line, line-and-plane, and plane-and-plane along with the key angles, completing the pure-geometry approach and connecting to the vector method ahead.