问些很笨的问题~~数学都不记得了~5555

tonyt913

NN们帮帮忙， 解答时能给出点具体公式和步骤吗？额比较笨，全都忘了~~

1. If n is positive integer and the product of all the integers from 1 to n, inclusive, is a multiple of 990, what is the least possible value of n?  a）10  b) 11  c) 12  d) 13

2. If a and b are positive numbers, what are the coordinates of the midpoint of line segment CD in the xy-plane
1)    The coordinates of C are (a, 1-b)
2)    The coordinates of D are (1-a, b)

3.  The residents of town x participated in a survey to determine the number of hours per week each resident spent watching television. The distribution of the results of the survey had a mean of 21 hours and a standard deviation of 6 hours. The number of hours that Pat, a resident of Town X, watched television last week was between 1 and 2 standard deviations below the mean. Which of the following could be the number of hours that Pat watched television last week?
a)    30  b)20  c)18  d)12  e)6
standard deviation below the mean - standard deviation能和mean不是两个东西吗？搞不太明白standard deviation

xubof117

不客气 ：）

tonyt913

明白了，谢谢ls两位~~

keanzhao

第一题：n是正整数，1到n的乘积是990的倍数，990可以分解成2 5 11 3 3，所以必须要有11，所以是最小B

第二题：（1/2 ,1/2)

第三题：不需要用到标准差的知识，楼上因该对的。

xubof117

第三题： 平均时间是21小时，标准差是6小时。上周看电视的时间在低于1-2个标准差之间，也就是说上周看电视的时间在 21-2*6=9 和 21-1*6= 15 之间，即： 9<x<15. 应该选D