8. When F moves on AD, EF 2 = AF 2+AE 2 = X 2+9X 2 = 10x 2, and the root number EF= 10x is linear; When F moves between DC, it will catch up with E before reaching C, that is, EF will be perpendicular to AB. At this time, the minimum value of EF is 3, and the rest is greater than 3, so the answer is C.

12, easy to know k=√3, let An- 1(xn- 1 0), an (xn, 0), Pn(x, √3/x). According to the relationship between the vertical line and the side length of an equilateral triangle, the side length of the nth triangle is 2/x Xn 2-xn-12 = 4, xn-12-xn-2 2 = 4, ... x22-x12.