AB=C
shape: A mxn , B nxp, C mxp
1、Cij=（row i of A）* (column j of B)
2、Matrix A times every column of B
A[b1 ... bp]=[Ab1 ... Abp]
3、Every row of A times matrix B
[row i of A] B= [row i of AB]
4、multiply column 1 to n of A times row 1 to n, add these matrices
AB= (column 1 of A ) (row 1 of B) +(column 2 of A)(row 2 of B) +... + (column n of A) ( row n of B)
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