Yi AD straight line y=-7√3x/9+ 10
Become a straight line y =-3 √ 3x/13-30 √ 3/13.
MN straight line y= 13√3x/9
Find m (3 √ 3/2, 13/2)
N(- 135/98，- 195√3/98)
The length of MN is 7+ 15√3/7.
Of course, this is only a method of clockwise rotation, and there should be another answer to counterclockwise rotation 7- 15√3/7.
Although I only used Pythagorean theorem in the last question of this kind of fill-in-the-blank problem, the calculation data is more complicated. After knowing the method, it takes 10 minutes to get the answer. Either there is a simpler method, or the examinee is asked to ignore this question. The point is, I don't know what level I have learned in junior high school mathematics now.