A=(a11a12a21a22)A=\begin{pmatrix} a_{11}& a_{12}\\ a_{21}& a_{22} \end{pmatrix}A=(a11a21a12a22),B=(b11b12b21b22)B=\begin{pmatrix} b_{11}& b_{12}\\ b_{21}& b_{22} \end{pmatrix}B=(b11b21b12b22) のとき, AB=(a11b11+a12b21a11b12+a12b22a21b11+a22b21a21b12+a22b22)AB=\begin{pmatrix} a_{11}b_{11}+a_{12}b_{21}& a_{11}b_{12}+a_{12}b_{22}\\ a_{21}b_{11}+a_{22}b_{21}& a_{21}b_{12}+a_{22}b_{22} \end{pm
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