题干

给出以下四个说法:

①绘制频率分布直方图时,各小长方形的面积等于相应各组的组距;

②在刻画回归模型的拟合效果时,相关指数R2的值越大,说明拟合的效果越好;

③设随机变量ξ服从正态分布N(4,22),则p(ξ>4)=

1
2

④对分类变量X与Y,若它们的随机变量K2的观测值k越小,则判断“X与Y有关系”的把握程度越大.

其中正确的说法是(   )

A:①④

B:②③

C:①③

D:②④

上一题 下一题 0.0难度 选择题 更新时间:2019-02-17 12:39:05

答案(点此获取答案解析)

B

同类题1

计算下列算式时,不用“七九六十三”这句口诀的是____。

①7×9    ②63÷7    ③7+9

同类题2

右图为铜锌原电池示意图,下列说法正确的是

同类题3

 

同类题4

若 AB→ =(2,4), AC→ =(1,3),则 BC→ =(  )

同类题5

阅读下列短文,根据短文内容选择最佳答案。

                                                                           C

    Flying is a fast way to travel. But it is a big headache for some people. They can't help worrying about things like, “Is turbulence (颠簸) a bad thing?” or, “Is it dangerous if we don't change our mobile phones to flight mode (飞行模式)?” British newspaper Daily Mail spoke to some experts to answer these questions. They also tell us other interesting things about flying.

    Is turbulence dangerous?

    It's scary and uncomfortable. But it is not something you really need to worry about.

    Turbulence is caused when two groups of air hit one another at different speeds. Wind, thunderstorms (暴风雨) and being close to mountains can be reasons for this.

    You may feel like turbulence is dangerous, but it won't crash (坠毁) your plane. “Even in extremely rough air (扰动气流), the wings (机翼) are not going to break off,” explained Patrick Smith, an airline pilot, who is also an air travel blogger and author of Ask The Pilot. com.

    Why are plane windows round?

    Round windows are for passenger safety.

    As the airplane flies higher and higher, the outside atmospheric pressure (气压) gets lower and lower. During the flight, the pressure inside the plane becomes greater than the pressure outside.

    When this happens, square windows can break. That's because having different pressures inside and outside puts pressure on the corners of square windows. But rounded corners can spread the pressure through the window.

    What happens when a phone isn't on flight mode?

    Some passengers believe mobile phones and tablets need to be turned off because they could bring the plane down. However, this is not true.

    If you don't change your phone to flight mode, pilots and air traffic controllers may hear unpleasant sounds in their headphones. It also influences radio communications. So it's still best to put it on flight mode.

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