A cylinder is intersected by a plane inclined at an angle of $60^{\circ }$ to its axis. What is the shape of the resulting cross-section?

When a right circular cylinder is sliced through by an inclined plane that is not perpendicular ($90^{\circ }$) or parallel ($0^{\circ }$) to its central axis, the resulting boundary profile of the planar cross-section forms a perfect ellipse.