The symbol is usedto showthat two lines,segments,or rays are perpendicular. In the precedingpicture,
we couldwrite. (Notethat is a ray while is a line.)
Notethat although"parallel"and "perpendicular"are definedin termsof lines,the samedefinitionsapplyto
rays and segmentswith the minoradjustmentthat two segmentsor rays are parallel(perpendicular)if the
lines that containthe segmentsor rays are parallel(perpendicular).
Example 1
Whichroadsare paralleland whichare perpendicularon the map below?The first step is to rememberthe definitionsor paralleland perpendicularlines.Parallellines lie in the same
planebut will neverintersect.Perpendicularlinesintersectat a right angle.All of the roadson this map lie
in the sameplane,and RoseAvenueand GeorgeStreetneverintersect.So, they are parallelroads.Henry
Streetintersectsboth RoseAvenueand GeorgeStreetat a right angle,so it is perpendicularto thoseroads.
Planescan be paralleland perpendicularjust like lines.Rememberthat a planeis a two-dimensionalsurface
that extendsinfinitelyin all directions.If planesare parallel,they will neverintersect.If they are perpendicular,
they will intersectat a right angle.