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\\[ \\begin{eqnarray*} AB &=& \\begin{pmatrix} \\var{a11}&\\var{a12}\\\\ \\var{a21}&\\var{a22}\\\\ \\end{pmatrix}\\begin{pmatrix} \\var{b11}&\\var{b12}\\\\ \\var{b21}&\\var{b22}\\\\ \\end{pmatrix}\\\\ &=& \\begin{pmatrix} \\simplify[]{{a11}{b11}+{a12}{b21}}&\\simplify[]{{a11}{b12}+{a12}{b22}}\\\\ \\simplify[]{{a21}{b11}+{a22}{b21}}&\\simplify[]{{a21}{b12}+{a22}{b22}}\\\\ \\end{pmatrix}\\\\ &=& \\begin{pmatrix} \\var{ab11}&\\var{ab12}\\\\ \\var{ab21}&\\var{ab22}\\\\ \\end{pmatrix} \\end{eqnarray*} \\]
\n\\[ \\begin{eqnarray*} BA &=& \\begin{pmatrix} \\var{b11}&\\var{b12}\\\\ \\var{b21}&\\var{b22}\\\\ \\end{pmatrix}\\begin{pmatrix} \\var{a11}&\\var{a12}\\\\ \\var{a21}&\\var{a22}\\\\ \\end{pmatrix}\\\\ &=& \\begin{pmatrix} \\simplify[]{{b11}{a11}+{b12}{a21}}&\\simplify[]{{b11}{a12}+{b12}{a22}}\\\\ \\simplify[]{{b21}{a11}+{b22}{a21}}&\\simplify[]{{b21}{a12}+{b22}{a22}}\\\\ \\end{pmatrix}\\\\ &=& \\begin{pmatrix} \\var{ba11}&\\var{ba12}\\\\ \\var{ba21}&\\var{ba22}\\\\ \\end{pmatrix} \\end{eqnarray*} \\]
\n\\[ \\begin{eqnarray*} CB &=& \\begin{pmatrix} \\var{c11}&\\var{c12}\\\\ \\var{c21}&\\var{c22}\\\\ \\end{pmatrix}\\begin{pmatrix} \\var{b11}&\\var{b12}\\\\ \\var{b21}&\\var{b22}\\\\ \\end{pmatrix}\\\\ &=& \\begin{pmatrix} \\simplify[]{{c11}{b11}+{c12}{b21}}&\\simplify[]{{c11}{b12}+{c12}{b22}}\\\\ \\simplify[]{{c21}{b11}+{c22}{b21}}&\\simplify[]{{c21}{b12}+{a22}{b22}}\\\\ \\end{pmatrix}\\\\ &=& \\begin{pmatrix} \\var{cb11}&\\var{cb12}\\\\ \\var{cb21}&\\var{cb22}\\\\ \\end{pmatrix} \\end{eqnarray*} \\]
\n\\[ \\begin{eqnarray*} AC &=& \\begin{pmatrix} \\var{a11}&\\var{a12}\\\\ \\var{a21}&\\var{a22}\\\\ \\end{pmatrix}\\begin{pmatrix} \\var{c11}&\\var{c12}\\\\ \\var{c21}&\\var{c22}\\\\ \\end{pmatrix}\\\\ &=& \\begin{pmatrix} \\simplify[]{{a11}{c11}+{a12}{c21}}&\\simplify[]{{a11}{c12}+{a12}{c22}}\\\\ \\simplify[]{{a21}{c11}+{a22}{c21}}&\\simplify[]{{a21}{c12}+{a22}{c22}}\\\\ \\end{pmatrix}\\\\ &=& \\begin{pmatrix} \\var{ac11}&\\var{ac12}\\\\ \\var{ac21}&\\var{ac22}\\\\ \\end{pmatrix} \\end{eqnarray*} \\]
", "metadata": {"licence": "Creative Commons Attribution 4.0 International", "description": "Multiplication of $2 \\times 2$ matrices.
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", "gaps": [{"correctAnswerFractions": false, "markPerCell": false, "type": "matrix", "showCorrectAnswer": true, "variableReplacements": [], "variableReplacementStrategy": "originalfirst", "allowResize": false, "marks": 1, "scripts": {}, "numRows": "2", "tolerance": 0, "correctAnswer": "matrix([\n [ba11,ba12],\n [ba21,ba22]\n])", "showFeedbackIcon": true, "numColumns": "2", "allowFractions": false}], "showCorrectAnswer": true, "type": "gapfill", "scripts": {}, "showFeedbackIcon": true, "variableReplacementStrategy": "originalfirst", "variableReplacements": []}, {"marks": 0, "prompt": "$CB = \\begin{pmatrix} \\var{c11}&\\var{c12}\\\\ \\var{c21}&\\var{c22}\\\\ \\end{pmatrix} \\begin{pmatrix} \\var{b11}&\\var{b12}\\\\ \\var{b21}&\\var{b22}\\\\ \\end{pmatrix}=$ [[0]]
", "gaps": [{"correctAnswerFractions": false, "markPerCell": false, "type": "matrix", "showCorrectAnswer": true, "variableReplacements": [], "variableReplacementStrategy": "originalfirst", "allowResize": false, "marks": 1, "scripts": {}, "numRows": "2", "tolerance": 0, "correctAnswer": "matrix([\n [cb11,cb12],\n [cb21,cb22]\n])", "showFeedbackIcon": true, "numColumns": "2", "allowFractions": false}], "showCorrectAnswer": true, "type": "gapfill", "scripts": {}, "showFeedbackIcon": true, "variableReplacementStrategy": "originalfirst", "variableReplacements": []}, {"marks": 0, "prompt": "$AC = \\begin{pmatrix} \\var{a11}&\\var{a12}\\\\ \\var{a21}&\\var{a22}\\\\ \\end{pmatrix}\\begin{pmatrix} \\var{c11}&\\var{c12}\\\\ \\var{c21}&\\var{c22}\\\\ \\end{pmatrix}=$ [[0]]
", "gaps": [{"correctAnswerFractions": false, "markPerCell": false, "type": "matrix", "showCorrectAnswer": true, "variableReplacements": [], "variableReplacementStrategy": "originalfirst", "allowResize": false, "marks": 1, "scripts": {}, "numRows": "2", "tolerance": 0, "correctAnswer": "matrix([\n [ac11,ac12],\n [ac21,ac22]\n])", "showFeedbackIcon": true, "numColumns": "2", "allowFractions": false}], "showCorrectAnswer": true, "type": "gapfill", "scripts": {}, "showFeedbackIcon": true, "variableReplacementStrategy": "originalfirst", "variableReplacements": []}], "statement": "Do the following matrix problems
Let
\\[A=\\begin{pmatrix} \\var{a11}&\\var{a12}\\\\ \\var{a21}&\\var{a22}\\\\ \\end{pmatrix},\\;\\; B=\\begin{pmatrix} \\var{b11}&\\var{b12}\\\\ \\var{b21}&\\var{b22}\\\\ \\end{pmatrix},\\;\\; C=\\begin{pmatrix} \\var{c11}&\\var{c12}\\\\ \\var{c21}&\\var{c22}\\\\ \\end{pmatrix}\\]
Calculate the following products of these matrices: