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标题: prep数学，求解 [打印本页]

作者: pianer 时间: 2010-1-4 21:33 标题: prep数学，求解

请大家看看，这两题怎么做呀？

34.E

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.

35.

If p and n are positive integers and p > n, what is the remainder when p^2 - n^2 is divided by 15 ?

(1) The remainder when p + n is divided by 5 is 1.

(2) The remainder

作者: pite82 时间: 2010-1-5 21:07

第一题

条件1虽然每桶水都变少了但是减少的程度是一样的就是说标准差没有变

条件2不能推出每桶变化的程度

作者: hellower 时间: 2010-1-6 21:43

1.（1）

计算过程：

实验后平均数=（1-30%）（t1+t2++t3+t4+t5+t6）/6=（1=30%）原平均数

实验后标准差=根号下【（（1-30%）t1-（1=30%）原平均数）^2+（（1-30%）t2-（1=30%）原平均数）^2+…………+（（1-30%）t6-（1=30%）原平均数）^2】/6

=（1-30%）原标准差

故条件（1）充分

（2）只给出实验后平均数显然无法确定标准差，不充分

故选a

2.（1）（2）单独显然不行

（1）（2）结合，相当于一个数被5除余1，另一个被3除余1，问两个数的积被15除的余数能不能确定，显然不行。举例：6×4=24，24被15除余9; 11×7=77，77被15除余2。故也不充分。

所以这题选e

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