单词拼写
   Spending time in a bookshop can be enjoyable. If you go to a good bookshop, no assistant will come near to you and say, “Can I 1h       you?” You needn’t buy anything you don’t want. You may try to find out 2w         the book you want is. If you fail(没有找到), the assistant will lead you there and 3t        he will go away. It seems that he is not 4i       in selling any books at all. There is a story about a good bookshop. A medical student 5f        a very useful book in the bookshop, but it was too 6e         for him to buy. He couldn’t get it from the library, either. So every afternoon, he went there to 7r        a little. One day, however, he couldn’t find the book from its 8u           place. He was leaving when he saw the assistant signing to 9h          . He was surprised that the assistant 10p           to the book in the corner. “I put it there so that people wouldn’t see it and buy it,” said the assistant. Then he left the student by himself to go on with his reading.

有下列四种说法：

①命题“∃x∈R，x²﹣x＞0”的否定是“∀x∈R，x²﹣x≤0”；

②“命题p∨q为真”是“命题p∧q为真”的必要不充分条件；

③“若am²＜bm²，则a＜b”的逆命题为真；

④若实数x，y∈0，1，则满足：x²+y²＜1的概率为$\frac{\mathrm{\pi }}{4}$．

其中错误的个数是  （　　）

用N_A表示阿伏加德罗常数的值，下列说法中正确的是（      ）