三阶矩阵A=（a1,a2,a3),B=(2a1,-a2,b),且|A|=1,|B|=-1,求|A-2B|

|A-2B|
= | (a1, a2, a3) - (4a1, -2a2, 2b)|
= |(-3a1, 3a2, a3-2b)|
= |(-3a1, 3a2, a3)| - |(-3a1, 3a2, 2b)|
= -9|A| - 9|B|
= 0
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|A-2B|
= |-3a1,3a2, a3-2b|
= -9 |a1,a2,a3-2b|
= -9(|a1,a2,a3| + |a1,a2,-2b|)
= -9(|A| + |2a1,-a2,b|)
= -9(|A|+|B|)

= 0.