阅读下面短文，根据短文内容判断句子的正误。

    More and more advertisements appear on the Web. You have to be careful not to be fooled or tricked(欺骗) by the advertisements on the Internet. Something is said to be a bargain(便宜物品), but it isn't always cheap. And some so-called new things just have a new outside.

    However, shopping online is a kind of fashionable(时尚的) thing today. I love to buy things of everyday use, so I often get lots of information about them on websites online. For example, if I want an MP3, I can choose my favorite one, and look for what the users like or don't like about it, then decide whether to buy or not. And at the same time, you don't have to go to a shop or walk around a crowded shopping area, so you don't have to waste much time. The Internet is really useful for shopping, but be careful not to spend too much.

中国的民族大家庭中，少数民族的数目共有(    )

若$0<\alpha <\frac{\mathrm{\pi }}{2},-\frac{\mathrm{\pi }}{2}<\beta <0,\cos \left(\frac{\mathrm{\pi }}{4}+\alpha \right)=\frac{1}{3},\cos \left(\frac{\mathrm{\pi }}{4}-\frac{\beta }{2}\right)=\frac{\sqrt{3}}{3}$，，则$\cos \left(\alpha +\frac{\beta }{2}\right)$=____