00:01
So in this question, they want us to prove that the identity 3x3 matrix times a matrix a is equal to said matrix a.
00:10
So in this example, we have a this 3x3 matrix, and the identity matrix for 3 by 3 is just ones on the diagonals and zeros everywhere else.
00:31
Now to do matrix multiplication, the identity times a, we take each row of the first matrix, which is the identity matrix in this case, and multiply it by every column of the second matrix, which is a.
00:54
So we take this row times this column, then times the second column, and the third column.
01:03
So we multiply 1 times negative 2, 0 times 3, and 0 times 0.
01:12
So of course that just gives us 1 times negative 2, which is negative 2.
01:15
Now we see when we do the first row times the second column, we are just taking the first entry of the second column...