⎡⎢⎣12101−13−11⎤⎥⎦A2=A.A=⎡⎢⎣12101−13−11⎤⎥⎦⎡⎢⎣12101−13−11⎤⎥⎦=⎡⎢⎣430−32−2645⎤⎥⎦A.A2=⎡⎢⎣12101−13−11⎤⎥⎦⎡⎢⎣430−32−2645⎤⎥⎦ = ⎡⎢⎣4111−9−2−721117⎤⎥⎦
⇒A3−3A2−A+9I3
= ⎡⎢⎣4111−9−2−721117⎤⎥⎦ −3⎡⎢⎣430−32−2645⎤⎥⎦ − ⎡⎢⎣12101−13−11⎤⎥⎦ +9⎡⎢⎣100010001⎤⎥⎦ = null matrix