求教初二数学难题已知:(1/x+1/y+1/z)^(1/3)=1/x+1/y+1/z,且xyz>0,求证:1/x+1/y
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求教初二数学难题
已知:(1/x+1/y+1/z)^(1/3)=1/x+1/y+1/z,且xyz>0,求证:1/x+1/y+1/z=1
已知:(1/x+1/y+1/z)^(1/3)=1/x+1/y+1/z,且xyz>0,求证:1/x+1/y+1/z=1
Let w=1/x+1/y+1/z. Then the given condition is w^3=w. So w=0 or 1 or -1. Since xyz>0, we have w>0. Hence w=1. This completes the proof.
再问: 非常感谢,但是—————————————————————————— 我就是搞不懂:凭xyz>0,为什么就可以得到1/x+1/y+1/z=1?为什么不是1/x+1/y+1/z=-1? 其实这道题是根据另一道很复杂的奥数题改编的。,最后就是不知道xyz>0和等于1有什么关系?
再答: Good question. I've misunderstood it before. In fact, xyz>0 implies either all the three variables are positive, or only one of them is positive and the other two are negative. In the former case, we deduce w>0 so w=1. In the latter case, for example, x=1 and y=z=-2 give w=0. Another example, x=1 and y=z=-1 give w=-1. So the original problem was wrong. The value of 1/x+1/y+1/z can be either 1 or -1 or 0.
再问: 谢谢,但是这段里我不认识的单词太多了,所以看不懂了。 我觉得由xyz>0,不如里面有两个负,一个正,可以有1/x+1/y+1/z=-1的可能呀?
再答: 是的。例如x=1 and y=z=-2 give w=0. 又例如x=1 and y=z=-1 give w=-1. 所以原题有误,0和正负1这三种情况都可能出现。
再问: 非常感谢,但是—————————————————————————— 我就是搞不懂:凭xyz>0,为什么就可以得到1/x+1/y+1/z=1?为什么不是1/x+1/y+1/z=-1? 其实这道题是根据另一道很复杂的奥数题改编的。,最后就是不知道xyz>0和等于1有什么关系?
再答: Good question. I've misunderstood it before. In fact, xyz>0 implies either all the three variables are positive, or only one of them is positive and the other two are negative. In the former case, we deduce w>0 so w=1. In the latter case, for example, x=1 and y=z=-2 give w=0. Another example, x=1 and y=z=-1 give w=-1. So the original problem was wrong. The value of 1/x+1/y+1/z can be either 1 or -1 or 0.
再问: 谢谢,但是这段里我不认识的单词太多了,所以看不懂了。 我觉得由xyz>0,不如里面有两个负,一个正,可以有1/x+1/y+1/z=-1的可能呀?
再答: 是的。例如x=1 and y=z=-2 give w=0. 又例如x=1 and y=z=-1 give w=-1. 所以原题有误,0和正负1这三种情况都可能出现。
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