若一个椭圆长轴的长度、短轴的长度和焦距成等差数列,则该椭圆的离心率是( )。
根据下面对话中的情景,在每个空白处填人适当的语句,使对话恢复完整。
A: Summer holiday is coming next month. What are you going to do?
B: We are going to Xi'an. 1
A: Oh, your grandparents must be very happy to see you again.
B: Of course they are.
A: 2
B: About four or five days.
A: 3
B: No, by train. Taking a bus is a little dangerous. 4
A: Maybe my parents will take Dick and me to some cities in the south.
B: That'll be great. 5
A: Maybe Hainan, Shenzhen and Guangdong.
B: Oh, They are all great cities. I think you'll have a good time.
A: You, too.
质量分别为m、2m、3m的物块A、B、C叠放在光滑的水平地面上,现对B施加一水平力F,已知AB间、BC间最大静摩擦力均为f0,为保证它们能够一起运动,F最大值为( )
A.6f0 B.4f0 C.3f0 D.2f0