设n维向量α=(12,0,…,0,12),矩阵A=E-αTα,B=E+2αTα,其中E为n阶单位矩阵,则AB=(  ) A.0 B.-E C.E D.E+αTα

问题描述:

设n维向量α=(

1
2
,0,…,0,
1
2
),矩阵A=E-αTα,B=E+2αTα,其中E为n阶单位矩阵,则AB=(  )
A. 0
B. -E
C. E
D. E+αTα


∵A=E-αTα,B=E+2αTα,
∴AB=(E-αTα)(E+2αTα)=E+2αTα-αTα-2αTααTα,
而:ααT=(

1
2
,0,…,0,
1
2
)
1
2
0
0
1
2
=
1
2

∴AB=E+2αTα-αTα-2αT(ααT)α=E+2αTα−αTα−2•
1
2
αTα
=E,
故选:C.