D in the second question is actually the distance from the center of the circle to ab minus the radius, right? The formula for this distance, which contains t, and then less than or equal to 1, calculates the range of t.

The third problem is to find two endpoints of y=kx, that is, two points where x equals 1 and-1. The distance between these two points and ab is D, and then discuss the scope according to the meaning of the question.