2. 外语
  3. If x and y are integers and xy3 is a positive odd integer, which of the following must be true?

If x and y are integers and xy3 is a positive odd integer, which of the following must be true?

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问题 If x and y are integers and xy3 is a positive odd integer, which of the following must be true?
选项 A、x+y is even.
B、xy is positive.
C、x2y is positive.
答案A,B
解析 条件：xy3是正奇数，可以拆解xy3＞0，且xy3是奇数。由xy3＞0，可以得出x和y3同号(同为正数或同为负数)，又y3与y同号，则x和y同号，那么B选项xy是正数，正确。因为x2＞0，那么y＞0时，x2y＞0；y＜0时，x2y＜0，不一定是正数，因此不选C选项x2y。又由xy3是奇数，得到x×y×y×y是奇数，那么x和y都是奇数。那么A选项x+y是偶数(奇数+奇数=偶数)，正确。所以答案选A和B。
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