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问题 单项选择题

设

，

，

，

，那么与对角矩阵相似的矩阵是______．

A．A，B，C
B．A，C，D
C．B，D
D．A，C

答案

参考答案：B

解析： 利用与对角阵相似的充分条件判别之，
易求得矩阵A的特征值是1，3，5．因为它们是三个不同的特征值，所以A～Λ．
矩阵B特征值是2，2，5，由于

，
所以，λ=2只有一个线性无关的特征向量，因而矩阵B不能相似对角化，
矩阵C是实对称矩阵，故必有C～Λ．
矩阵D的特征值也是2，2，5．由于秩

，
所以，λ=2有两个线性无关的特征向量，因而矩阵D可以相似对角化．仅B入选．

单项选择题

女，52岁，活动后心悸气急2年，一月来症状加重，有夜间阵发性呼吸困难及下肢浮肿，心电图示：QRS时限≥0.12秒，V1～V2导联呈rsR，R波粗顿；V5一V6导联呈qRS，S波宽阔，T波与QRS主波方向相反，最可能的心电图诊断是：

A．正常心电图

B．窦性心律不齐

C．预激综合征

D．左束支传导阻滞

E．右束支传导阻滞
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单项选择题

Amusement rides at permanent fairgrounds are dismantled once a year for safety inspections by independent consultants. Traveling fairs, which relocate each month, can slip past the net of safety inspections and escape independent inspection for several years. Therefore, the rides at traveling fairs are less safe than the rides at permanent fairs.
Which of the following, if true about traveling fairs, most seriously weakens the argument

A．Before each relocation, the operators dismantle their rides, observing and repairing potential sources of danger, such as worn ball bearings.

B．Their managers have less capital to spend on the safety and upkeep of the rides than do managers of permanent fairs.

C．Since they can travel to new customers, they rely less on keeping up a good reputation for safety.

D．While they are traveling, the fairs do not receive notices of equipment recalls sent out by the manufacturers of their rides.

E．(E) The operators of the rides often do-not pay careful attention to the instructions for operating their rides.
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