First, change the "n" in N rows to the end. Do you need to move it? n- 1？ Time, that is, (-1) (n- 1), and then change all the elements in line k to 1. Let this new determinant be |B|, then Ak 1+Ak2+...+Akn=|B|.

On the solution of |B|;

This should be clear!