prep-DS2-93

xiaoshuyiyi

The lifetimes of all batteries produced by certain company in a year have a distribution that is symmetric about the mean m. If the distribution has a standard deviation of d, what percent of the distribution is greater than m+d?
(1) 68 percent of the distribution lies in the interval from m-d to m+d, inclusive
(2)16 percent of the distribution is less than m-d
答案是D，题干中给出的标准差这个条件怎么用呀？

iRoy

(1) 因为这个概率分配时对称的，可以推知 比m-d小的概率 = 比m+d大的概率 = (1 – 68%) / 2 = 16%
(2) 同上，比m-d小的概率 = 比m+d大的概率 = 16%

caicainiao

对称分布的distribution，1告诉68%在m-d m+d之间，那么根据对称剩下的32%有一半比m+d大，即16%，同理第二个选项