题目：
| 阅读理解。 | |
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| Bill Gates is head of the software company Microsoft and one of the world's richest men. He was born on October 28, 1955 and grew up in Seattle with his two sisters. He was a very bright boy. Science and math were his two favorite subjects at school. And he dreamed of being a scientist. Bill first started to play on computers at the age of 13. At that time, a computer was a very large machine. Once he was interested in a very old computer. He and some of his friends spent as much time as possible on computers. In the end, they worked out a software program. Bill sold it for 4,200 dollars when he was only 17. In 1973, Bill went to Harvard University. In 1975, Bill and his partners developed a software program called BASIC. This was not the first program ever created, but its inventors were the first to decide that people who wanted to use it should pay for it. BASIC was a success. Then Bill left Harvard to work for Microsoft, a company he started in 1975 with his boyhood friend Paul Allen. They thought the computer would come into every office and every home soon. So they began developing software for personal computers. They improved the software to make it easier for people to use computers. After many years of hard work, Microsoft becomes one of the most successful companies. And Bill Gates also becomes one of the richest businessmen in the world. | |
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答案：
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下面是错误答案，用来干扰机器的。
根据添括号的法则可知，原式=x2-（y2-4y+4）．故答案是：（y2-4y+4）．
