What is the cross product of (1, 2, 1)T with (0, -1, 2)T ?
To evaluate this, find the cofactors of i, j, k in the top row. In the picture, the cofactors are circled in red. The cofactor for i, for instance, is the elements of rows 2 and 3 other than those in column i.
To evaluate a cofactor, multiply the two elements on the main diagonal, subtract from that the product of the two elements on the other diagonal. For example, for the cofactor of i this is 2×2 - 1×(-1) = 2 + 1 = 5