请教两道PREP数学题

3. 196-!-item-!-187;#058&000141

How many different prime numbers are factors of the positive integer n ?

(1) Four different prime numbers are factors of 2n.

(2) Four different prime numbers are factors of n^2.

答案 B 但是我觉得（1）也充分

3. 196-!-item-!-187;#058&000141

How many different prime numbers are factors of the positive integer n ?

(1) Four different prime numbers are factors of 2n.

(2) Four different prime numbers are factors of n^2.

答案 B 但是我觉得（1）也充分

34  2927-!-item-!-187;#058&003024

During an experiment, some water was removed from each of 6 water tanks. If the standard deviation of the volumes of

water in the tanks at the beginning of the experiment was 10 gallons, what was the standard deviation of the volumes of

water in the tanks at the end of the experiment?

(1) For each tank, 30 percent of the volume of water that was in the tank at the beginning of the experiment was removed

during the experiment.
(2) The average (arithmetic mean) volume of water in the tanks at the end of the experiment was 63 gallons.

谢谢先！

第一题，举反例，

n=2*3*5*7

n=3*5*7

conditon 1 is not sufficient

第二题，我选A 对吗？

我只算出A了，第二个条件没算出来

谢谢楼上MM

第二题能说说A是怎么算的么？意见standard deviation我就蒙了

第一题：

看N是奇数还是偶数了，不一样的，所以A不行的

第二题：

首先知道标准差的公式，呃，太难编辑了，LZ自己去查查吧，很容易查得到的。

如果全体数据都乘以某一个系数，那么平均数也自然乘以这个系数，所以可以从平方里提出，再从根号里提出来，是可以计算新方差的。

第二个条件，没说原来的平均数是多少，用不上，所以选A