Question

in a set all 2×3 matrices specify the additive identity
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Answer 1

The additive identity matrix would be a zero matrix (all its entries are zeros.)

The sum between two matrices can be done if and only if the two matrices are similar, that means that they have the same numbers of rows and columns.

Also the additive identity matrix has to be similar to the other, so every shape of matrix has its identity matrix.

The elements, obviously, are all zeros.

E.G.

The matrix with the shape: 3 rows and 4 columns has this identity matrix:

000

000