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(a)

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\\[ \\begin{eqnarray*} Av &=& \\begin{pmatrix} \\var{a11}&\\var{a12}&\\var{a13}\\\\ \\var{a21}&\\var{a22}&\\var{a23}\\\\ \\var{a31}&\\var{a32}&\\var{a33}\\\\ \\end{pmatrix}\\begin{pmatrix} \\var{v1}\\\\ \\var{v2}\\\\ \\var{v3}\\\\\\end{pmatrix}\\\\ &=& \\begin{pmatrix} \\simplify[]{{a11}{v1}+{a12}{v2}+{a13}{v3}}\\\\ \\simplify[]{{a21}{v1}+{a22}{v2}+{a23}{v3}}\\\\ \\simplify[]{{a31}{v1}+{a32}{v2}+{a33}{v3}}\\\\ \\end{pmatrix}\\\\ &=& \\begin{pmatrix} \\var{p1}\\\\ \\var{p2}\\\\ \\var{p3}\\\\ \\end{pmatrix} \\end{eqnarray*} \\]

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\\[ \\begin{eqnarray*} Bw &=& \\begin{pmatrix} \\var{b11}&\\var{b12}&\\var{b13}\\\\ \\var{b21}&\\var{b22}&\\var{b23}\\\\ \\var{b31}&\\var{b32}&\\var{b33}\\\\ \\end{pmatrix}\\begin{pmatrix} \\var{w1}\\\\ \\var{w2}\\\\ \\var{w3}\\\\\\end{pmatrix}\\\\ &=& \\begin{pmatrix} \\simplify[]{{b11}{w1}+{b12}{w2}+{b13}{w3}}\\\\ \\simplify[]{{b21}{w1}+{b22}{w2}+{b23}{w3}}\\\\ \\simplify[]{{b31}{w1}+{b32}{w2}+{b33}{w3}}\\\\ \\end{pmatrix}\\\\ &=& \\begin{pmatrix} \\var{q1}\\\\ \\var{q2}\\\\ \\var{q3}\\\\ \\end{pmatrix} \\end{eqnarray*} \\]

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\\[ \\begin{eqnarray*} AB &=& \\begin{pmatrix} \\var{a11}&\\var{a12}&\\var{a13}\\\\ \\var{a21}&\\var{a22}&\\var{a23}\\\\ \\var{a31}&\\var{a32}&\\var{a33}\\\\ \\end{pmatrix}\\begin{pmatrix} \\var{b11}&\\var{b12}&\\var{b13}\\\\ \\var{b21}&\\var{b22}&\\var{b23}\\\\ \\var{b31}&\\var{b32}&\\var{b33}\\\\ \\end{pmatrix}\\\\ &=& \\begin{pmatrix} \\simplify[]{{a11}{b11}+{a12}{b21}+{a13}{b31}}&\\simplify[]{{a11}{b12}+{a12}{b22}+{a13}{b32}}&\\simplify[]{{a11}{b13}+{a12}{b23}+{a13}{b33}}\\\\ \\simplify[]{{a21}{b11}+{a22}{b21}+{a23}{b31}}&\\simplify[]{{a21}{b12}+{a22}{b22}+{a23}{b32}}&\\simplify[]{{a21}{b13}+{a22}{b23}+{a23}{b33}}\\\\ \\simplify[]{{a31}{b11}+{a32}{b21}+{a33}{b31}}&\\simplify[]{{a31}{b12}+{a32}{b22}+{a33}{b32}}&\\simplify[]{{a31}{b13}+{a32}{b23}+{a33}{b33}}\\\\ \\end{pmatrix}\\\\ &=& \\begin{pmatrix} \\var{ab11}&\\var{ab12}&\\var{ab13}\\\\ \\var{ab21}&\\var{ab22}&\\var{ab23}\\\\ \\var{ab31}&\\var{ab32}&\\var{ab33}\\\\ \\end{pmatrix} \\end{eqnarray*} \\]

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(b)

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An $m\\times n$ matrix has $m$ rows and $n$ columns

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If A is an $m \\times n$ matrix and B is a $p \\times q$ matrix then we can calculate AB only if $n =p$, in which case AB is a $m \\times q$ matrix.

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In other words, we can only multiply matrices if number of columns of first matrix = number of rows of second matrix

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Elementary Exercises in multiplying matrices. 

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Let

\n

\\[A = \\left(\\begin{array}{rrr} \\var{a11} & \\var{a12} & \\var{a13}\\\\ \\var{a21} & \\var{a22} & \\var{a23}\\\\ \\var{a31} & \\var{a32} & \\var{a33}\\\\ \\end{array}\\right),\\;\\;\\;\\; B= \\left(\\begin{array}{rrr} \\var{b11} & \\var{b12} & \\var{b13}\\\\ \\var{b21} & \\var{b22} & \\var{b23}\\\\ \\var{b31} & \\var{b32} & \\var{b33}\\\\ \\end{array}\\right),\\;\\;\\;\\; v= \\left(\\begin{array}{r} \\var{v1}\\\\ \\var{v2} \\\\ \\var{v3} \\end{array}\\right),\\;\\;\\;\\; w= \\left(\\begin{array}{r} \\var{w1}\\\\ \\var{w2} \\\\ \\var{w3} \\end{array}\\right)\\]

\n

Find the following products:

\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
\\[ Av=\\left( \\begin{matrix} \\phantom{.}\\\\ \\phantom{.}\\\\ \\phantom{.}\\\\ \\phantom{.}\\\\ \\end{matrix} \\right.\\][[0]]\\[\\left) \\begin{matrix} \\phantom{.} \\\\ \\phantom{.}\\\\ \\phantom{.}\\\\ \\phantom{.}\\\\ \\end{matrix} \\right.\\]
[[1]]
[[2]]
\\[ Bw=\\left( \\begin{matrix} \\phantom{.}\\\\ \\phantom{.}\\\\ \\phantom{.}\\\\ \\phantom{.}\\\\ \\end{matrix} \\right.\\][[3]]\\[\\left) \\begin{matrix} \\phantom{.} \\\\ \\phantom{.}\\\\ \\phantom{.}\\\\ \\phantom{.}\\\\ \\end{matrix} \\right.\\]
[[4]]
[[5]]
\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
\\[AB=\\left( \\begin{matrix} \\phantom{.}\\\\ \\phantom{.}\\\\ \\phantom{.}\\\\ \\phantom{.}\\\\ \\end{matrix} \\right.\\][[6]][[7]][[8]]\\[\\left) \\begin{matrix} \\phantom{.} \\\\ \\phantom{.}\\\\ \\phantom{.}\\\\ \\phantom{.}\\\\ \\end{matrix} \\right.\\]
[[9]][[10]][[11]]
[[12]][[13]][[14]]
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$CD$

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$DC$

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$EF$

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$FE$

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$BC$

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$AE$

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$GH$

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$HE$

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$AG$

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$GB$

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Consider the following matrices together with the matrices from the first part of the question.

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\\[\\begin{eqnarray}&C=& \\var{mac},\\;\\;\\;\\; &D=& \\var{mad},\\;\\;\\; \\;&E= &\\var{mae}\\\\&F=& \\left(\\begin{array}{rr} \\var{w1} & \\var{a12}\\\\ \\var{w2} & \\var{b23} \\\\ \\var{w3} & \\var{w2} \\\\\\var{v1} & \\var{b12}\\\\ 0 & \\var{-w2} \\end{array}\\right),\\;\\;\\;\\;&G=&\\var{mag},\\;\\;\\;\\;&H=&\\var{mah} \\end{eqnarray}\\]

\n

Which of the following products of matrices can be calculated?

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[[0]]

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Please note that for every correct answer you get 0.5 marks and for every incorrect answer 0.5 is taken away. The minimum mark you can get is 0.

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Answer the following questions on matrices.

\n

 

", "type": "question", "contributors": [{"name": "Newcastle University Mathematics and Statistics", "profile_url": "https://numbas.mathcentre.ac.uk/accounts/profile/697/"}, {"name": "Simon Thomas", "profile_url": "https://numbas.mathcentre.ac.uk/accounts/profile/3148/"}]}]}], "contributors": [{"name": "Newcastle University Mathematics and Statistics", "profile_url": "https://numbas.mathcentre.ac.uk/accounts/profile/697/"}, {"name": "Simon Thomas", "profile_url": "https://numbas.mathcentre.ac.uk/accounts/profile/3148/"}]}