问题如下：
On Black Monday in October 1987, the Dow Jones Indus-trial Average fell from the previous close of 2,246.74 to 1,738.74. What was the simple return on this day? What was the log return?
选项：
解释：
The simple return is (1738.74 - 2246.74)/ 2246.74= -22.6%. The
log return is ln 1738.74 - ln 2246.74 = -25.6%.
The difference between the two is increasing in the magnitude.
请问log return is ln 1738.74 - ln 2246.74 = -25.6%.这个公式是哪里来的？
