I have exam for uni entrance tomorrow and have essentially done 0 work on conic sections, one of the possible subjects. Am I on the right track with this?
Assuming a cone formed from the line z=ax, T in origo. Call the axis of symmetry S. Call the angle between S and the conic line (l:ax) v. Call the angle between the "cutting" plane and the axis of symmetry θ. Call the curve formed by intersecting points of the "cutting plane" and the cone the "curve of intersection"
Start of explanation:
Assume a circle is formed by the curve of intersection. If so, then the z-coordinate of all points on the curve of intersection must be constant. in this case θ is obviously = 0 deg. Assume circle had focal points A and B in its center. If B is moved "up" the z-axis, i.e. l, an ellipse will form. This is obviously true for all 90 deg> θ>v (v is the angle between S and l) . When the angle reaches θ=v, then the plane must be parallel to l, meaning B is at infinity. The parabola is an ellipse with one focal point at infinity.