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【简答题】
设总体x～N(户I，d2)。总体y—N(2，仃2)，设X,，X2，…，Xn,是来自 总体X的样本，设Y,，Y2，…，y。是来自总体y的样本，两样本独立． (1)求参数u 1 -u 2 的一个无偏估计； (2)证明： 是σ 2 的无偏 估计，这里 分别是两样本的均值
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举一反三
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A.
a; a
B.
the; the
C.
a; /
D.
the; /
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A.
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B.
match
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defend
D.
Fight
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B.
He had to close one of the shops.
C.
His second shop didn't open on time.
【简答题】能量信号的特点?
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【简答题】信号按信号的能量特点进行分类：
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