CK12 - Geometry

A. ray

B. line

C. segment

D. plane

Sincethe straightroad connectstwo distinctpoints(cities),and we are interestedin the sectionbetween
thosetwo endpoints,the best term issegment. A segmenthas two endpoints.So, the correctansweris C.

Example 4

Whichterm best describesthe relationshipamongthe stringson a tennisracket?

(Source:http://commons.wikimedia.org/wiki/File:Tennis_Racket_and_Balls.jpg;License:GNUFree Docu-
mentation)

A. collinear

B. coplanar

C. non-collinear

D. non-coplanar

The stringsof a tennisracketare like intersectingsegments.Theyalso are all locatedon the planemade
by the headof the racket.So, the best answeris B. Note that the stringsare not reallythe sameas segments
and they are not exactlycoplanar, but we can still use the geometricmodelof a planefor the headof a
tennisracket,evenif the modelis not perfect.

BasicPostulates

Nowthat we havesomebasicvocabulary, we can talk aboutthe rulesof geometry. Logicalsystemslike
geometrystart with basicrules,and we call thesebasicrulespostulates.We assumethat a postulateis
true and by definitiona postulateis a statementthat cannotbe proven.

Atheoremis a statementthat can be proventrue usingpostulates,definitions,logic,and othertheorems
we’vealreadyproven.Theoremsare the “results”that are true givenpostulatesand definitions.This section
introducesa few basicpostulatesthat you mustunderstandas you moveon to learnothertheorems.

The first of five postulatesyou will studyin this lessonstatesthat thereis exactlyone line throughany two
points.You couldtest this postulateeasilywith a ruler, a pieceof paper, and a pencil.Use your pencilto
drawtwo pointsanywhereon the pieceof paper. Use your ruler to connectthesetwo points.You’ll find that
thereis only one possiblestraightline that goesthroughthem.