FindMissingDimensionsofRectangles

LessonPlannerObjectiveFindthemissingdimensionofarectangleorsquarewhenotherinformationabouttheshapeisgiven.

Usethislessonafterlesson11-6andExtend11-6toaddressstandardM.TE.04.07andM.TE.04.08.

Materials: gridpaper,indexcards

ActivatePriorKnowledgeInpreviouslessons,studentslearnedtheareaandperimeterformulasofrectangles.• Askstudentstorecalltheformulaforthe

areaofarectangle. A=�×w• Thenaskstudentstorecalltheformula

fortheperimeterofarectangle.P=�+�+w+w

UsingstudentpageMI36.Studentswillneedtousetheareaandperimeterformulasofrectanglestofindmissingdimensionsofrectangles.• Passoutindexcardscontainingthelength,

width,area,orperimeterofarectangletostudents(oneperstudent).Makesurethatonelengthorwidthcardmatcheswithoneareaorperimetercard.Forexample,someofthecardsmatchesmaybe:l=3inchesandA=12squareinches;w=5centimetersandP=18centimeters.

• Oncestudentsfindtheirmatch,theymustworktogethertofindthemissinglengthorwidthoftheirrectangle.Tellstudentstouseeithertheareaorperimeterformulaofarectangletofindthemissingdimension.

• Discusstheanswersandhowtheywerefoundasaclass.

• WorkthroughtheteachandExamplesonstudentpage36.

UsingtheExercisesExercises 7–8 RemindstudentsthattheareaformulaofasquareisA=s×sandthattheycanusetheperimeterformulaofarectangletofindtheperimeterofasquare.

AssessandCloseAnother Dimension Havestudentsexplaininwritinghowtofindthemissingdimensionofarectanglethathasalengthof7inchesandanareaof42squareinches.

ForExercises1-8,seeAnswerAppendixp.MI38.

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Name

Find Missing Dimensions of RectanglesYoucanfindthemissingmeasureofarectangleifyouknowthemeasureoftheotherside.

1 Find the width of the rectangle shown.

Usetheareaformulaofarectangletofinditswidth.

A=��×w36=9×w36=9×4

So,thewidthis4inches.

2 Find the length of the rectangle shown.

Usetheperimeterformulaforarectangletofinditslength.

P=(2×��)+(2×w)28=(2×��)+(2×6)28=(2×��)+1228=2×8+12

So,thelengthis8centimeters.

EXAMPLES

Areaformulaofarectangle

Replace�with9.

Think:9�=36?

PerimeterformulaofarectangleReplacewwith6.

Multiply

Think:2�+12=28?

MI4-001A-112372

9 in.

? in.

Area = 36 sq in.

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6 cm

Perimeter = 28 cm

? cm

36 MapforSuccess

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The area and the measure of one side of each rectangle is given. Find the length of the missing side.

1.

MI4-003A-112372

3 mm

Area = 12 sq mm

? mm 2.

MI4-004A-112372

6 cm

Area = 18 sq cm

? cm 3.

MI4-005A-112372

4 yd

Area = 28 sq yd

? yd

The perimeter and the measure of one side of each rectangle is given. Find the length of the missing side.

4.

MI4-006A-112372

8 in.

Perimeter = 20 in.

? in. 5.

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3 ft

Perimeter = 24 ft

? ft 6.

MI4-008A-112372

6 mm

Perimeter = 32 mm

? mm

Exercises

8. Whatisthemeasureofthesidesofasquarethathasaperimeterof36inches?

7. Whatisthemeasureofthesidesofasquarethathasanareaof49squarefeet?

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