CK12 - Geometry

The last undefinedterm isplane. You can thinkof a planeas a hugesheetof paper—sobig that it goeson
forever!Imaginethe paperas thin as possible,and extendit up, down,left, and right.Planescan be named
by letter, or by threepointsthat lie in the plane.You alreadyknowone planefrom your algebraclass—the

-coordinateplane.Planesare calledtwo-dimensional,sinceany pointon a planecan be describedby
two numbers,calledcoordinates,as you learnedin algebra.

NotationNotes: As new termsare introduced,notationnoteswill help you learnhow to write and say them.

1.Pointsare namedusinga singlecapitalletter. The first imageshowspoints , , and.

2.

In the imageof a line, the sameline has severalnames.It can be called“line ”, , or. The

orderof the lettersdoesnot matterwhennaminga line, so the sameline can havemanynames.When

usingtwo pointsto namea line, you mustuse the line symbol abovethe letters.

3. Planesare namedusinga script(cursive)letteror by namingthreepointscontainedin the plane.The
   illustratedplanecan be calledplane or “the planedefinedby points , , and .”

Example 1

Whichterm best describeshow San Diego,California,wouldbe representedon a globe?

A. point

B. line

C. plane

A city is usuallylabeledwith a dot, or point,on a globe.Thoughthe city of San Diegooccupiesspace,it is
reducedwhenplacedon the globe.Its labelis merelyto showa locationwith referenceto the othercities,
states,and countrieson a globe.So, the correctansweris A.

Example 2

Whichgeometricobjectbest modelsthe surfaceof a moviescreen?

(Source:http://commons.wikimedia.org/wiki/File:Airscreen.JPG;License:GNUFree Documentation)

A. point

B. line