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Lecture 8: Rules for Matrix Math 2270 Operations

Math2270-Lecture8 ,itshouldbemostlyareview, ,asitisthefoundationforeverythingelsewe llbedoinginthiscourse!Amatrixisarectangu lararrayofnumbers,andan mbyn Matrix ,alsowrittenrnxn, , :/23\/35\/58(34)+(10Hf\i2)\\23)\\35/12\/ 36313 I=191224)\ ,becauseitisn ,iftwomatriceshavethesamesizeandshape,it ,it ,wecouldnot,forexample, ,however, ,thentheproductABwillbeamatrixwiththenum berofrowsinA, ,forexample, , (AB)3=(rowiofA).(columnjofB).So,asanexam ple,forthematrices)B=(2 TheproductBAdoesnotmakesense,buttheprodu ctABdoes, +B=B+Ac(A+B)=cA+cBA+(B+C)=(A+B)+CC(A+B)= CA+CB(A+B)C=AC+BCA(BC)=(AB)CPerhapsthemo stinteresting,andunexpected,oftheaboveru lesisA(BG)=(AB) ,andthatmatrixmultiplicationisassociativ eisn tobviousfromthedefinitionofhowmatricesar emultiplied,butit , , (12(21N(6521)22)64while(21\(12N(45(65(12 N(2 22)21)65)64)21)221 WeassumethroughoutthatA,B, (A+B)2isdifferentfromA2+2AB+B2,whenandB= (j)IiT/-(c)(o)(d/Io_170(odL3)-()(I/IL-(, f31(A=(Writedownthecorrectrulefor(A+B)(A +B)=A2+ABi9A_+ (30//1(5(L31I3)(3)IC)-3c--/ioc)AM+()(Y// /Yo(oot[7 43 MatrixPowersWeca)))))))]

work for matrix multiplication. The usual rules for exponents, namely = P+ and (AP) = still apply. We define A° = I, where I is the identity matrix of the same size as A. We also define A’ to be the inverse of A, so A3would be A’A’A’. Note that in usual arithmetic the inverse of a number exists unless the num ber is zero.

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Transcription of Lecture 8: Rules for Matrix Math 2270 Operations

1 Math2270-Lecture8 ,itshouldbemostlyareview, ,asitisthefoundationforeverythingelsewe llbedoinginthiscourse!Amatrixisarectangu lararrayofnumbers,andan mbyn Matrix ,alsowrittenrnxn, , :/23\/35\/58(34)+(10Hf\i2)\\23)\\35/12\/ 36313 I=191224)\ ,becauseitisn ,iftwomatriceshavethesamesizeandshape,it ,it ,wecouldnot,forexample, ,however, ,thentheproductABwillbeamatrixwiththenum berofrowsinA, ,forexample, , (AB)3=(rowiofA).(columnjofB).So,asanexam ple,forthematrices)B=(2 TheproductBAdoesnotmakesense,buttheprodu ctABdoes, +B=B+Ac(A+B)=cA+cBA+(B+C)=(A+B)+CC(A+B)= CA+CB(A+B)C=AC+BCA(BC)=(AB)CPerhapsthemo stinteresting,andunexpected,oftheaboveru lesisA(BG)=(AB) ,andthatmatrixmultiplicationisassociativ eisn tobviousfromthedefinitionofhowmatricesar emultiplied,butit , , (12(21N(6521)22)64while(21\(12N(45(65(12 N(2 22)21)65)64)21)221 WeassumethroughoutthatA,B, (A+B)2isdifferentfromA2+2AB+B2,whenandB= (j)IiT/-(c)(o)(d/Io_170(odL3)-()(I/IL-(, f31(A=(Writedownthecorrectrulefor(A+B)(A +B)=A2+ABi9A_+ (30//1(5(L31I3)(3)IC)-3c--/ioc)AM+()(Y// /Yo(oot[7 43 MatrixPowersWecantakepowersofmatrices,bu tonlyifthey ,thenA ,namely=P+and(AP))))))))]

2 = =I, tobetheinverseofA,soA3wouldbeA A A . , ,A3,A4for0200A 0020 00020000o1Oo\oof0zoOQ0Oo00C-)0C)a0A3o0l0 /0oLQJ-00//0 IQoZ(000 Qooc))LA /0LQ(QQ\ o0000oc00000Ac(15cme/2 JjI.


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