I know that this question was asked over a year ago and that no one will read this post. But I can't sit idly by and watch all these wrong answers get posted.
The if a and b were real or complex numbers, then the equation a^2 - b^2 = (a + b)(a - b) would be true, for the reasons posted above. But a and b are matrices, not real numbers. Implicit in all of the calculations above is that ab = ba. But that doesn't hold for most pairs of matrices.
So for example, if you take
a =
1 2
3 4
b =
5 6
7 8
You'll find you get
a^2 - b^2 =
-60 -68
-76 -84
and yet
(a + b)(a - b) =
-56 -56
-88 -88
The if a and b were real or complex numbers, then the equation a^2 - b^2 = (a + b)(a - b) would be true, for the reasons posted above. But a and b are matrices, not real numbers. Implicit in all of the calculations above is that ab = ba. But that doesn't hold for most pairs of matrices.
So for example, if you take
a =
1 2
3 4
b =
5 6
7 8
You'll find you get
a^2 - b^2 =
-60 -68
-76 -84
and yet
(a + b)(a - b) =
-56 -56
-88 -88