67.
Yes. By the definition of inverse, if AB=I, then BA=I and A and B are inverses.
(A−1)−1=A
75. If A is invertible, then I=A−1A=A−1(AB)=(A−1A)B=I B=B.
77.
A−1=[3−2−43]
79. {(1, 2, 1, 2)}
81.
A=[86108]
A=[−1−3717]
A=⎡⎣⎢⎢32521232⎤⎦⎥⎥
A=⎡⎣−152−24⎤⎦
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