请教一个到现在没搞明白的数学题

teyvhf

一列数,mean是m,一倍标准差(还是方差)是n,已知68%的数字分布within m and n,问多少percent的数字位于<M+N。答案是84%

我的哈佛

The only thing you need to look at is the graph of standard deviation,


the shape will tell you immediately what is the trick played here in this question.


The line in the middle is called expectation, which in reality, equals mean for short.


Then the downs of the graph add up to 100%


Did you see what I mean?