$\sqrt{3}$的倒数为____ ．

如图所示，一价氢离子（ ${}_1^1$ H）和二价氦离子（ ${}_2^4$ He）的混合体，经同一加速电场U₁同时加速后，垂直射入同一偏转U₂电场中，偏转后，打在同一荧光屏上，则它们（   ）

阅读理解，判断正误。

Bill and Ben are shopping.

Bill: There is a store. Let's go in.

Ben: Let's go.

Assistant: Can I help you?

Bill: Yes, do you have any shoes?

Assistant: Yes, we do. What size do you want?

Bill: Size 21, please.

Assistant: Here you are.

Bill: Oh, this shoe is just right for my left foot. But this one is too big for my right foot.

Assistant: You can try this pair on. They are size 20.

Bill: Oh, this time my right foot is all right. But this shoe is too small for my left foot.

Assistant: What size do you want?

Bill: I don't know. Can you give me an idea?

This book ____   ____.