- #1

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

1. (In-A)(In+A) = In - A^2

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

3. A^8*B^5 is invertible

4. (A+A^-1)^9 = A^9 + A^-9

5. AB = BA

6. A+A^-1 is invertible

I know 5 is right, and number 2 wrong, but what else?