题目内容：

设n维行向量α=（1/2,0,...,0,1/2）,矩阵A=E-α'α,B =E+2α'α,则AB=
AB
= (E-α'α)(E+2α'α)
= E + α'α - 2α'αα'α
= E + α'α - 2α'(αα')α
= E + α'α - 2α' (1/2)α//这步不太懂
= E