Determine which of the following formulas hold for all invertible nxn matrices A and B?

1. A+B is invertible

2. (I - A)(I + A) = I - A^2

3. A^9 is invertible

4. (A + A^-1)^3 = A^3 + A^-3

5. AB = BA

6. (A+B)^2 = A^2 + B^2 + 2AB

I originally thought that 4 was the only one not applicable, but apparently that's incorrect.