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Your writing in your post is INCORRECT,
THEREFORE, I edited it in couple of lines below.
Suppose A,B and AB-I are non-degenerate and have same order. Prove that
is non-degenerate;
is non-degenerate. Find the inverse of this matrix
This problem was posted to the forum several years ago, and I solved it under this link
https://www.algebra.com/algebra/homework/Matrices-and-determiminant/Matrices-and-determiminant.faq.question.1099395.html
Below I copy-pasted that solution here again for your convenience.
..............................
Since the matrix B is non-degenerated, it has the inverse matrix .
Multiply the matrix by the matrix on the right. You will get
= .
Since both matrices on the left side of the equation are non-degenerated, their product on the right side is non-degenerated, too.
QED
Solved.