标准差的公式不是除以N-1吗？怎么这里不用除啊

问题如下：

The probability distribution for a company’s sales is:

The standard deviation of sales is closest to which of the following?

选项：

A.

USD9.81 million

B.

USD12.20 million

C.

USD32.40 million

解释：

A is correct. The analyst must first calculate expected sales as 0.05 × USD70 +0.70 × USD40 + 0.25 × USD25 = USD3.50 million + USD28.00 million + USD6.25 million = USD37.75 million.

After calculating expected sales, we can calculate the variance of sales:

σ² (Sales)= P(USD70)[USD70 – E(Sales)]² + P(USD40)[USD40 – E(Sales)]² + P(USD25)[USD25 – E(Sales)]²

= 0.05(USD70 – 37.75)² + 0.70(USD40 – 37.75)² + 0.25(USD25 – 37.75)²

= USD52.00 million + USD3.54 million + USD40.64 million= USD96.18 million.

The standard deviation of sales is thus σ = (USD96.18)^1/2 = USD9.81 million.