28.2-1 slightly simpler solution

[jxu]

If we square the matrix [[A B] [0 0]], I think we arrive at a similar result [[A^2 AB] [0 0]]

[jxu]

Same with [[0 A] [B 0]]. the result is [[AB 0] [0 AB]]

[walkccc]

Please feel free to open a PR.