for simplicity factor a out of the matrix and then add row 3 to both row 1 and row 2 to get
Now expand down the third column to get
The question:
Show that the following matrix is not invertible
My attempt:
I know that when the determinate is 0, a matrix is not invertible. So I did the following:
=
Which becomes:
=
But this isn't 0 like required. :/
What have I done wrong? Thanks.