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Homework Statement
28. Let A be an m x n matrix with a row consisting entirely of zeros. Show that if B is an n x p matrix, then BA has a row of zeros.
Homework Equations
N/A
The Attempt at a Solution
[itex]A = (aij)_{mxn}[/itex] and [itex]B = (bij)_{nxp}[/itex]. Assuming that the entries for jth column of A are all zeros, I infer that the jth column of BA is also a row of zeros. Let [itex]cij[/itex] be an entry in jth column. By the definition of multiplication:
[tex] cij = b_{i1}a_{j1} + b_{i2}a_{j2} + ...+ b_{in}a_{nj} = \sum_{k=1}^n b_{ik}a_{kj}[/tex]
Since the jth column of A is zero, then there is:
[itex]a_{1j} = a_{2j} = ... = a_{nj} = 0[/itex]. Hence, [itex]cij = 0[/itex] and, therefore, the jth column of BA is a column of zeros.
Did I do this proof correctly? Thanks in advance.
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