2. 外语
  3. The standard deviation of n numbers x1, x2, x3, …, xn with mean is equal to , where S is the sum of the squared differences (x1

The standard deviation of n numbers x1, x2, x3, …, xn with mean is equal to , where S is the sum of the squared differences (x1

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问题 The standard deviation of n numbers x1, x2, x3, …, xn with mean is equal to , where S is the sum of the squared differences (x1-)2 for 1≤i≤n. If the standard deviation of the 4 numbers 5-a, 5, 25, and 25+a is 50, where a＞0, what is the value of a?
选项
答案60
解析 通过观察发现5-a，5，25，25+a这四个数是“对称”的，平均值是15，则根据标准差公式：

200+2(a+10)2=10000
(a+10)2=4900
a+10=70或-70(舍去)
a=60
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