The coordinate system is established with any point on the central axis of the cylinder as the origin, because the conclusion that the oblique section of the cylinder is elliptical is established under restrictive conditions. Then cut diagonally through this origin, and then prove it. Whether it is in the form of an equation or the sum of distances, it can be concluded that it is elliptical. The premise of all this is flawed. Just grasp this and prove it.

Another idea is projection. The projection of the cross section must be a circle, and the projected plane is either a parallel circle or an unparallel ellipse. The oblique section is not parallel to the bottom of the cylinder, so it must be an ellipse.