请教！！谢谢！

lileelee

1. How much longer is the edge of a cube with a surface area of 1350 than the edge of a cube with a surface area of 600?
Answer: 5

2.        The probability that event A occurs is 0.6 and the probability that event B occurs is 0.5. What is the greatest value of the probability that events A and B both not occur?
     答案 0.4， 为什么不是0.2？

3． A list has 100 numbers, the expectation of the list is 10 and the standard deviation of the list is 4, then which pair of the integers would make the standard deviation of the list decrease when the integers are put into the list?
     答案 (10, 10)

LittleLilyLiAng

1。求表面积为1350的正方体的边长比表面积为600的正方体的边长长多少。设边长分别为a,b，则有6a^2=1350,6b^2=600,得a=15,b=10.所以,所求为a-b=5.
2.我想,此题画一个文式图可以很清楚的解释.已知了A,B各自单独发生的概率以后,求A,B同时都不发生的最大概率.要想A,B都不发生的概率最大,就是说要使A+B以外的部分最小,只有在B包含在A中的时候能够实现.所以,这时A和B都不发生的概率就是1-0.6=0.4.
3.这题涉及数学期望,标准差什么的,学过了太久,有些生疏了,我再回忆一下,再解释吧.有哪位高手如能回答,望不吝赐教.谢谢!

dancingforum

expectation 就是算术平均, mean 值
标准方差 = 开平方(数列中每个数与mean的差值的平方和/N)

所以,(10,10) 不改变分子(数列中每个数与mean的差值的平方和), 但是 N 变成了 N +１
so standard deviation will decrease