定义区间（a，d），[a，d），（a，d]，[a，d]的长度为d﹣a（d＞a），已知a＞b，则满足 1x−a+1x−b≥1 的x构成的区间的长度之和为_1．_刷题宝

题干

定义区间（a，d），[a，d），（a，d]，[a，d]的长度为d﹣a（d＞a），已知a＞b，则满足 −

1

x

a

+−

1

x

b

≥1 的x构成的区间的长度之和为____．

上一题下一题 0.0难度选择题更新时间:2016-09-08 05:34:08

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2

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给出下列四个命题：

①由样本数据得到的回归方程 $\hat{y} = \hat{b} x + \hat{a}$ 必过样本点的中心（ $\bar{x}$ ， $\bar{y}$ ）；

②用相关指数R²来刻画回归效果，R²的值越小，说明模型的拟合效果越好；

③若线性回归方程为 $\hat{y}$ =3﹣2.5x，则变量x每增加1个单位时，y平均减少2.5个单位；

④在残差图中，残差点分布的带状区域的宽度越窄，残差平方和越小．

上述四个命题中，正确命题的个数为（）

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下列四组动物中，都属于昆虫的是（）

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小刚每天都是7∶25上学，他要提前1小时起床，提前30分钟吃早点．小刚____时起床？____时吃早点？

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建国后，我国的人口政策从“人多力量大”一一鼓励生育，“只生一个好”一一计划生育，到“单独二孩”一一政策放宽，再到“全面二孩”的调整。这表明（）

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