先化簡,再求值:
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,其中

=3.
解:
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=
=

---------------2分
=
=
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. ----------------------4分
當

時,原式=
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=
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.解析:
略
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對于試題:“先化簡,再求值:
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解:
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=x-3-(x+1)
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

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
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
=

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
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