gre数学辅导：基本概念及实例解析(4)
gre数学考试在新gre考试当中增加了一些难度，但是gre考生也不要太紧张，只要把一些gre数学基本概念掌握好，解题时就一定能轻松应对。一元二次方程ax2+bx+c=o
x12=[-b±(b2-4ac1/2)]/2a
描述统计 Descriptive statistics
mode 众数
一组数中出现频率最高的数
极差 range
一组数中最大值与最小值之间的差值
平均数 mean
算术平均 arithmetic mean
几何平均 geometric mean
例题.The circumference of a rectangle is 20
A.The greatest area of the rectangle
B.25
中位数 median
中位数的寻找方法：
1.将所有n个数字按着从大到小或者从小到大排列；
2.如果n=奇，整个序列中位于中间位置的数就是中位数；如果n=偶，中间两个数的算术平均即是中位数。
例题.If the arithmetic mean of a, b, c, 3 and 5 is 6, which of the following must be true?
1.The median of the five numbers cannot be 3
2.At least one of a, b and c is greater than 7
3.The range of the five numbers is 2 or more
A.1 only
B.2 only
C.3 only
D.1 and 3
E.2 and 3
例题.A group of students were tested on data-entry skills, and the number of errors they made and the percent of students making x errors were recorded as follows.
Number of errors
Percent of students
0
1%
1
7%
2
8%
3
25%
4
19%
5
18%
6 or more
22%
1. What is the range for the number of errors made by the students in the sample?
A. 4
B.5
C.6
D.7
E. It cannot be determined from the information given.
2. What was the median number of errors in the sample?
A.3
B.3.5
C.4
D.4.5
E. It cannot be determined from the information given.
平时，gre考生一定要注意gre数学基本概念的加强理解，因为无论gre数学考试怎样的题目都是从最基本的概念中演变而来的。
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