抛物线

A parabola is a conic section formed when a plane intersects a double-napped cone parallel to the cone's slant height. It is defined as the set of all points equidistant from a fixed point called the focus and a fixed line called the directrix. This geometric relationship creates the characteristic open curve used in structures like suspension bridges and satellite dishes.

Every point on a parabola maintains equal distance to the focus and directrix. The axis of symmetry passes through the vertex and focus, perpendicular to the directrix. The vertex is the point equidistant from both the focus and directrix, serving as the pivot for the curve's symmetry and determining whether the parabola opens upward, downward, left, or right.

The standard form depends on axis orientation. For a vertical axis with vertex at (h, k), the equation is (x - h)² = 4p(y - k). For a horizontal axis, it is (y - k)² = 4p(x - h). Here, p is the distance from vertex to focus. When the vertex is at the origin, these simplify to y² = 4px and x² = 4py respectively.

The parameter p indicates both the distance from vertex to focus and the opening direction. If p is positive, the parabola opens upward or rightward toward the focus. If p is negative, it opens downward or leftward. The focus always lies in the direction the parabola opens, opposite the directrix across the vertex.

The standard form emerges by applying the distance formula to equate distances from any point to the focus and directrix. Squaring both sides eliminates square roots. Expanding and simplifying the resulting expressions removes common terms, revealing the standard form. Swapping x and y variables yields equations for different axis orientations.

The reflective property of parabolas states that the tangent at any point makes equal angles with the line to the focus and the axis of symmetry. This property results from the law of reflection and the curve's geometric symmetry, making parabolas ideal for applications like satellite dishes and headlights that focus parallel rays.

Moving the vertex from the origin to point (h, k) modifies the standard form by replacing x with (x - h) and y with (y - k). This translation shifts the entire parabola horizontally and vertically while preserving its shape, focus-directrix relationship, and opening direction. The parameter p remains unchanged.