6．設(shè)A是n階方陣，若對任意的n維向量X均滿足AX=0，則(      )

A．A=0                                                     

B．A=E

C．秩(A)=n                                               

D．0<秩(A)<n

7．設(shè)矩陣A與B相似，則以下結(jié)論不正確的是(      )

A．秩(A)=秩(B)                                         

B．A與B等價(jià)

C．A與B有相同的特征值                         

D．A與B的特征向量一定相同
8．設(shè)為矩陣A=的三個(gè)特征值，則=(      )

A．10                                                        

B．20

C．24                                                        

D．30

9．二次型f(x₁，x₂，x₃)=的秩為(      )

A．1                                                         

B．2

C．3                                                          

D．4

10．設(shè)A，B是正定矩陣，則(      )

A．AB一定是正定矩陣                             

B．A+B一定是正定矩陣

C．(AB)^T一定是正定矩陣                          

D．A-B一定是負(fù)定矩陣