CK12 - Geometry

C. plane

The surfaceof a moviescreenextendsin two dimensions:up and downand left to right.This description
mostcloselyresemblesa plane.So, the correctansweris C. Notethat a planeis a modelof the movie
screen,but the screenis not actuallya plane.In geometry, planesextendinfinitely, but the moviescreen
doesnot.

DefinedTerms

Now we can usepoint,line, andplaneto definenew terms.One wordthat has alreadybeenusedisspace.
Spaceis the set of all pointsexpandingin threedimensions.Thinkbackto the plane.It extendedalongtwo
differentlines:up and down,and side to side.If we add a third direction,we havesomethingthat lookslike

three-dimensionalspace.In algebra,the planeis adaptedto modelspaceby addinga third axis
comingout of the page. The imagebelowshowsthreeperpendicularaxes.

Pointsare said to becollinearif they lie alongthe sameline. The picturebelowshowspoints , , and

are collinear. Point isnon-collinearwith the otherthreesinceit doesnot lie in the sameline.

Similarly, pointsand lines can becoplanarif they lie withinthe sameplane.The diagrambelowshowstwo

lines ( and ) and one point( ) that are coplanar. It also showsline and point that are

non-coplanarwith and.