1.

Give an example of twonon-zero `2xx2`matrices `A`and `B`such that `A B=O`.

Answer» Let `A=[{:(1,1),(1,1):}]" and "B=[{:(" "1,-1),(-1," "1):}]`. Then,
`AB=[{:(1,1),(1,1):}][{:(" "1,-1),(-1," "1):}]`
`=[{:(1-1,-1+1),(1-1,-1+1):}]=[{:(0,0),(0,0):}]`
and `BA=[{:(" "1,-1),(-1," "1):}][{:(1,1),(1,1):}]`
`=[{:(" "1-1," "1-1),(-1+1,-1+1):}]=[{:(0,0),(0,0):}].`
Thus, `AneO, BneO.`
But, AB=BA=O.


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