Libraries

    Public Libraries

    Most towns in Britain have a public library.A library usually has a large selection of books and other resources,which library members can use and borrow for free.

    Britain's First Public Library

    The first public library in Britain opened in Manchester in 1852.Its first librarian was a man called Edward Edwards.Edwards attended the library's opening ceremony and two famous writers called Charles Dickens and William Thackeray were there too.

    How to Join a Library

    To join a library,go to your local library and fill in a form.You'll receive a library card which is needed when using library services.

    Library Services

    Modem public libraries have something for everyone.Most of them have audio books,CDs,newspapers and DVDs as well as books to borrow.

    Mobile Libraries

    Not everyone can get to a library.Some people live far away from towns and cities.Other people find it difficult to go out because they have an illness or a disability.Thanks to mobile libraries,these people can still borrow books.

    Unlike most libraries,which store books in buildings,mobile libraries usually keep their books in a mini-bus.The back of the mini-bus has shelves for the books,and it is big enough for borrowers to step inside and look around.A driver take the mini-bus to a certain place at a certain time,so people know when to expect it.They can return their books and borrow some more.

    All Aboard the Library!

    A school in London didn't have space for a library inside the building.However,everybody agreed that it was still important to have a school library.So,the head of the school,Graham Blake,decided to park an old bus on the school car park and change it into a library.Pupils helped to paint the bus,and after eight months,the new library was ready.

为了防止电路中电流过大，发生危险，电路中常常要安装保险丝，保险丝是用 ____大，熔点低的铅锑合金制作的，新建楼房用空气开关代替了保险丝，如图所示，当电路中电流过大时，电磁铁的磁性将变 ____（选填“强”或“弱”），吸引衔铁，使衔铁转动，闸刀在弹力作用下自动开启，切断电路，起到保险作用．

《初中科学实验活动册》中有一个如图所示的实验：在大试管里装满水，然后把小试管（小试管的直径略小于大试管）放到大试管中，一起倒过来，这时我们观察到水向下流，小试管却向上运动．该实验可以证明（　　）

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（Ⅰ）由折线图可以看出，可用线性回归模型拟合月度市场占有率y与月份代码x之间的关系．求y关于x的线性回归方程，并预测M公司2017年4月份的市场占有率；

（Ⅱ）为进一步扩大市场，公司拟再采购一批单车．现有采购成本分别为1000元/辆和1200元/辆的A、B两款车型可供选择，按规定每辆单车最多使用4年，但由于多种原因（如骑行频率等）会导致车辆报废年限各不相同．考虑到公司运营的经济效益，该公司决定先对两款车型的单车各100辆进行科学模拟测试，得到两款单车使用寿命频数表如下：

经测算，平均每辆单车每年可以带来收入500元．不考虑除采购成本之外的其他成本，假设每辆单车的使用寿命都是整数年，且以频率作为每辆单车使用寿命的概率．如果你是M公司的负责人，以每辆单车产生利润的期望值为决策依据，你会选择采购哪款车型？

参考数据：， $\sum _{i=1}^6\left(x_i-\overline{x}\right)\left(y_i-\overline{y}\right)=35$ ， $\sum _{i=1}^6{\left(x_i-\overline{x}\right)}^2$ =17.5．

参考公式：

回归直线方程为 $\hat{y}=\hat{b}x+\hat{a}$ 其中 $\widehat{b}$ = $\frac{{\sum }_{i=1}^n\left(x_i-\overline{x}\right)\left(y_i-\overline{y}\right)}{{\sum }_{i=1}^n{(x_i-\overline{x})}^2}$ ， $\widehat{a}$ = $\widehat{y}$ ﹣ $\widehat{b}\overline{t}$ ．