The original picture is an arc (also an image) in the first quadrant of a circle with the center of (3, -2) and the radius of root 13. The graph rotates counterclockwise around the origin, and the locus of the center is a circle with the origin as the center and the root number 13 as the radius (marked as circle c). When the C and Y axes of the circle are tangent to the coordinate origin, it is a critical situation, and the counterclockwise rotation is no longer the image of the function. In this process, the point (3,2) turns to the point (root13,0), so the angle is arctangent (2/3).

Do you understand? (I am a math teacher in senior three)