Let A and B be square matrices of the same size. Prove that if A and B are both upper triangular, then AB is upper triangular. If A and B are both upper triangular, do they necessarily commute? If you think they must commute, give a proof; if you think they might not commute, give a counterexample.

Elementary Linear Algebra (MindTap Course List)
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Chapter3: Determinants
Section3.CR: Review Exercises
Problem 43CR: Let A and B be square matrices of order 4 such that |A|=4 and |B|=2. Find a |BA|, b |B2|, c |2A|, d...
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Let A and B be square matrices of the same size.

Prove that if A and B are both upper triangular, then AB is upper triangular.

If A and B are both upper triangular, do they necessarily commute? If you think they must commute, give a proof; if you think they might not commute, give a counterexample.

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