Another geometric way to define the conics is as the locus of all points in the plane whose distances, r, from a fixed point called the focus, and a, from a given straight line called the directrix, have a constant ratio. This ratio, r/a, is known as the eccentricity, e. The circle has an eccentricity of zero. As the eccentricity increases from near zero, corresponding to a nearly-circular ellipse, the ellipse stretches until the right-hand side of it disappears to infinity, e becomes 1, and the ellipse turns into a parabola, with just one open branch. Like the circle, the parabola has only one shape, though it may look different depending on how much it is enlarged or diminished. As the eccentricity increases beyond 1, the "lost" right-hand end of the ellipse reappears from the other side of infinity, so to speak, and turns into the left-hand branch of a hyperbola.
Because a hyperbola is effectively an ellipse split in two by infinity, it comes as no surprise that these curves are related in an inverse way. An ellipse consists of all points whose distances from two foci have a constant sum, while a hyperbola is made from all points whose distances from two foci have a constant difference. These definitions also apply to the circle and the parabola, if the two foci are considered to coincide in the case of the circle and to be separated by an infinite distance in the case of the parabola.
In terms of algebra, the family of conics represents all the possible real number solutions to the general quadratic equation ax2 + bxy + cy2 + dx + ey + f = 0. In other words, the graph of any quadratic with real solutions is always a conic section. The key quantity is the difference b2 - 4ac. If this is less than zero, the graph is an ellipse, a circle, a point, or no curve. If b2 - 4ac = 0, the graph is a parabola, two parallel lines, one line, or no curve; if it is greater than zero, the graph is a hyperbola or two intersecting lines.
Conic sections also represent all the possible orbits an object can follow when under the gravitational influence of a single massive body.
Related categories PLANE CURVES
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