已知a是有理数，有下列判断：①a是正数；②-a是负数；③a与-a必有一个是负数；④a与-a互为相反数，其中正确的有______个．
∵a可能是正数、也可能是0，还可能是负数，同样-a可能是正数、也可能是0，还可能是负数，①错误；②错误；
∵当a=0时，a和-a都是0，都不是负数，∴③错误；
∵不论a是正数、0负数，a与-a都互为相反数，∴④正确；
即正确的有1个，
故答案为：1．
Some people have the ideas that nothing can be done about their poor reading ability．They feel hopeless about it．Can you learn to read better, or must you agree that nothing can be done about it? To be sure, people’s abilities are different．You cannot expect to do everything as well as certain other people do．If all the students in a class tried out for basketball, some would be very good players; others would be very poor; and many would be in between．But even the very poor players can become much better players if they are guided in the right way, and with plenty of practice．It is the same with reading．Some seem to enjoy reading and to read well without any special help．Others find reading a slow and tiring job．In between, there  are all degrees of reading ability．Many experiments have shown that just about every poor reader can improve his reading ability．Firstly, the poor readers were given tests of reading ability．Secondly, some of the causes of their poor reading were discovered, and then they were given special instruction and practice in reading．After a few months, another test of the same kind was given．In nearly all cases, these people had raised their reading scores．
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