(a)
A2B = (AA)B = A(AB) = A(BA) = (AB)A = (BA)A = B(AA) = BA2
(b)
A2 - 3A + I = 0
A2 - (3I)A + I = 0
A-1[A2 - (3I)A + I) = A-1(0)
A-1A2 - A-1[(3I)A] + A-1I = 0
A-1(AA) - [A-1(3I)]A + A-1 = 0
A scalar multiple of the identity commutes with all other matrices.
(A-1A)A - [(3I)A-1]A + A-1 = 0
IA - (3I)[A-1A] + A-1 = 0
A - (3I)I + A-1 = 0
A - 3I + A-1 = 0
A = 3I - A
(d)
is the coefficient matrix of this system for some p,q,r:
if =4, the system is
This is inconsistent unless r=0
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if =6, the system is
This is inconsistent unless p=0
Edwin