题目

  People __61__ (live) in different countries made different kinds of words. Today there are about fifteen hundred __62__ in the world. Each language contains many thousands of words. A very large dictionary, for example, contains four __63_ five hundred thousand words. But we do not need _64_ these. To read short stories you need to know only about two thousand words. _65_ you leave school, you will learn only one thousand or more. The words you know are called your vocabulary. You should try to make your vocabulary __66_ (big). Read as many books as we can. There are a lot of books 证明对任意的正整数n，都有13+23+33+…+n3=.