For a matrix like this we have to select the matrix size from min(r,c) where r and c are row and column sizes . For ex in the image below we will have to find det of 3×3 matrix from it i.e select any 3 column of your choice and proceed.
Important: We can only multiply matrices if the number of columns in the first matrix is the same as the number of rows in the second matrix.
a) Multiplying a 2 × 3 matrix by a 3 × 4 matrix is possible and it gives a 2 × 4 matrix as the answer.
b) Multiplying a 7 × 1 matrix by a 1 × 2 matrix is okay; it gives a 7 × 2 matrix
c) A 4 × 3 matrix times a 2 × 3 matrix is NOT possible.
