问题 解答题

四个不同的小球放入编号为1,2,3,4的四个盒中,则恰有一个空盒的放法有多少种?

答案

144

先将4个小球分成4份,其中一份有2个小球,一份有0个小球,另两个各是一份,有种不同的分组方法,再将这4份放到4个不同的盒子中,有种不同的放法.

共有6×24=144种不同的放法.

名师点金:在排列组合综合问题中,一般是先选后排,先分组后排序,注意分组时,若是平均分组,则应注意组数之间的顺序问题,如上面的解答中,剩下的两个小球分成两组,若采用算法,则将分成的两组之间排了一次顺序,因此还要除以两组之间的排列.

阅读理解

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小题1: Why do you think online bookstores are much better than traditional ones?

A.Because online bookstores offer readers the latest books.

B.Because online bookstores offer readers more kinds of books to choose from.

C.Because online bookstores offer readers the lowest prices.

D.Because online bookstores offer readers more about their favorite authors.小题2: How can a reader buy a worthy book when shopping online?

A.By comparing service.

B.By asking for discounts.

C.By reading online reviews

D.By asking booksellers for advice.小题3:What do online book lovers care about most?

A.The speed of deliveries

B.The author of books.

C.The prices of books.

D.The condition of books.小题4: The wish list tool can help readers        .

A.learn about the number of the books

B.get to know more reliable booksellers

C.pay for their books more quickly

D.buy their favorite books in time小题5:What is the 3rd paragraph mainly about?

A.How to choose a reliable online bookstore.

B.How to avoid buying bad condition books online.

C.How to understand the specialization of an online bookstore.

D.How to advertise on the website to get a good reputation.

单项选择题