1. 都是实数，没有虚数
2. 10进制，除非另外说明。
3. billion是10的9次方，dozen是12个，π是3.14近似
4. 23.5近似到个位是24, 123.985近似到百分位是123.99,36.5近似到个位是37
5. 循环小数的循环节上画一个横线。
6. factor因子，可正可负，因子一般都是整数，倍数不一定
7. 最小公倍数和最大公约数
8. 余数、被除数、除数、商的定义
9. 质数的定义
10. 奇数偶数不一定是正数
11. 0既不是正的，也不是负的
12. 小于等于、大于等于、约等于、不等于、绝对值、平方根、阶乘、平行、垂直等的符号需要知道
13. 没有定义的情况：分母为0,根号下面为负数，0的0次方
14. 有时候，字母摆在一起不表示乘法，比如consider a three-digit integer denotedby XYZ, where X, Y, and Z are digits.
15. 如果一个函数的定义域没有明说，那么就默认是整个实数范围，复合函数会考。
16. 只考欧几里德平面，三角形内角和180
17. 直线没有端点
18. 多边形是凸的，闭合的
19. 解析几何一般都是按照比例画的
20. non-empty sets, empty set,intersection, union, disjoint sets or mutually exclusive sets
21. the integers from 0 to 9, inclusive。 包括了10个元素，而during the years from 1985 to 2005 不管说不说inclusive都包括了21年
22. measures of central tendency,measures of position, measures of dispersion
23. 如果有偶数个数，那么median表示按照升序排列的这串数字的中间两个数的算术平均
24. mode可以不只一个
25. quartile可以指那个点，也可以指一个区，比如the cow’sweight is in the third quartile of the weights of the herd 这个时候quartile 就是指一个区域group
26. 25th percentile = the firstquartile; the 50th percentile equals the second quartile, or median; the 75thpercentile equals the third quartile.
27. standard deviation of the datarefers to the nonnegative square root of the mean of the squared differencesbetween m and each of the data. also known as the population standard deviationand is not to be confused with the sample standard deviation. Sample standarddeviation is divided by n-1 for technical reasons.
28. frequency distribution
29. relative frequency distributions
30. the mean of the random variable X isthe sum of the products XP(X) for all values of X.
31. the mean of a random variable X isalso called the expected value of X or the mean of the probability distributionof X
32. In any group of data, most of thedata are within about 3 standard deviations above or below the mean.
33. The process of subtracting the meanfrom each value and then dividing the result by the standard deviation iscalled standardization. It's a useful tool because for each data value, itprovides a measure of position relative to the rest of the data independentlyof the variable for which the data was collected and the units of the variable.
34. universal set
35. median 把曲线下面的面积平均分成两块面积相等的部分，mean要算上这块面积的重量，所以og上那个图的mean要在median的右边，因为越往右，越重
36. 所以median是halving point, 而mean is the balance point
37. box-and-whisker plot 5个点L Q1 Q2 Q3 G 可以立刻看到median, interquartile range and range.
38. 正态分布：2%，14%，34%，34%，14%，2%