=√(〖"[" 2"-(" 1"-" t")]" 〗^2+〖"[" t"-(" 1"-" t")]" 〗^2+〖"(" t"-" t")" 〗^2 )

　　=√(〖"(" 1+t")" 〗^2+〖"(" 2t"-" 1")" 〗^2+0)

　　=√(5t^2 "-" 2t+2)=√(5(t"-" 1/5)^2+9/5)≥(3√5)/5,

　　所以A,B两点间距离的最小值是(3√5)/5.

答案：C

5如图,在正方体ABCD-A'B'C'D'中,棱长为1,点P在对角线BD'上,且BP=1/3BD',则点P的坐标为(　　)

A.(1/3 "," 1/3 "," 1/3) B.(2/3 "," 2/3 "," 2/3)

C.(1/3 "," 2/3 "," 1/3) D.(2/3 "," 2/3 "," 1/3)

解析：点P在坐标平面xDy上的射影落在BD上.

　　因为BP=1/3BD',所以Px=Py=2/3,Pz=1/3.

　　故点P的坐标为(2/3 "," 2/3 "," 1/3).

答案：D

6在空间直角坐标系中,若点P在x轴上,它到P1(0,√2,3)的距离为2√3,则点P的坐标为　　　　　.

解析：设P(x,0,0),则√(x^2+2+9)=2√3,解得x=±1,故P点坐标为(±1,0,0).

答案：(±1,0,0)

7在空间直角坐标系中,已知点A(1,0,-2),B(1,-3,1),点B关于坐标平面xOy的对称点为B1,则|AB1|=　　　　.

答案：√10