對于任意實數
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,下列五個命題中:
①若
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,則
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;
②若
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,則
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;
③若
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,則
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;
④若
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則
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;
⑤若
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,則
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.
其中真命題的個數是( )
試題分析:
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,當
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時,
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不成立,①是假命題;
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,當
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時,
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不成立,②是假命題;
因為
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,所以,
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,
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,③是真命題;
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當
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同號時,
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成立,而
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異號時,
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不成立,④是假命題;
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時,
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不一定成立,只有當
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時,
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成立,⑤是假命題。
故選A.
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