2. 外语
  3. For any numbers a and b, ab=ab(3-b). If a and ab both represent positive numbers, which of the following could be a value of b?

For any numbers a and b, ab=ab(3-b). If a and ab both represent positive numbers, which of the following could be a value of b?

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问题 For any numbers a and b, ab=ab(3-b). If a and ab both represent positive numbers, which of the following could be a value of b?
选项 A、-5
B、-2
C、2
D、4
E、7
答案C
解析 The most direct approach to this sort of problem is to plug in the answers and see which one works. Choices A and B are out because any negative value for b would result in a negative value for ab if a is positive. C works because it gives the result ab=2a, which has to be positive. D and E do not work because they result in negative values for ab. Therefore, C is the only answer that provides a possible value for b.
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