Question

suppose that B is a matrix whose third column is the sum of the first two columns, then the third column of AB is
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Answer 1

Answer:

The column third of B is sum of previous two columns of B. It means row vector of B satisfies the linear relation. So, we can say that the matrix's rows AB are all linear combination of the rows of B which is satisfies this lines relation. So, column third is the same combination of previous two columns of AB.