# 7.5 Conic sections

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• Identify the equation of a parabola in standard form with given focus and directrix.
• Identify the equation of an ellipse in standard form with given foci.
• Identify the equation of a hyperbola in standard form with given foci.
• Recognize a parabola, ellipse, or hyperbola from its eccentricity value.
• Write the polar equation of a conic section with eccentricity $e$ .
• Identify when a general equation of degree two is a parabola, ellipse, or hyperbola.

Conic sections have been studied since the time of the ancient Greeks, and were considered to be an important mathematical concept. As early as 320 BCE, such Greek mathematicians as Menaechmus, Appollonius, and Archimedes were fascinated by these curves. Appollonius wrote an entire eight-volume treatise on conic sections in which he was, for example, able to derive a specific method for identifying a conic section through the use of geometry. Since then, important applications of conic sections have arisen (for example, in astronomy), and the properties of conic sections are used in radio telescopes, satellite dish receivers, and even architecture. In this section we discuss the three basic conic sections, some of their properties, and their equations.

Conic sections get their name because they can be generated by intersecting a plane with a cone. A cone has two identically shaped parts called nappes . One nappe is what most people mean by “cone,” having the shape of a party hat. A right circular cone can be generated by revolving a line passing through the origin around the y -axis as shown. A cone generated by revolving the line y = 3 x around the y -axis.

Conic sections are generated by the intersection of a plane with a cone ( [link] ). If the plane is parallel to the axis of revolution (the y -axis), then the conic section    is a hyperbola. If the plane is parallel to the generating line, the conic section is a parabola. If the plane is perpendicular to the axis of revolution, the conic section is a circle. If the plane intersects one nappe at an angle to the axis (other than $90\text{°}\right),$ then the conic section is an ellipse. The four conic sections. Each conic is determined by the angle the plane makes with the axis of the cone.

## Parabolas

A parabola is generated when a plane intersects a cone parallel to the generating line. In this case, the plane intersects only one of the nappes. A parabola can also be defined in terms of distances.

## Definition

A parabola is the set of all points whose distance from a fixed point, called the focus    , is equal to the distance from a fixed line, called the directrix    . The point halfway between the focus and the directrix is called the vertex    of the parabola.

A graph of a typical parabola appears in [link] . Using this diagram in conjunction with the distance formula, we can derive an equation for a parabola. Recall the distance formula: Given point P with coordinates $\left({x}_{1},{y}_{1}\right)$ and point Q with coordinates $\left({x}_{2},{\phantom{\rule{0.2em}{0ex}}\text{y}}_{2}\right),$ the distance between them is given by the formula

$d\left(P,Q\right)=\sqrt{{\left({x}_{2}-{x}_{1}\right)}^{2}+{\left({y}_{2}-{y}_{1}\right)}^{2}}.$

Then from the definition of a parabola and [link] , we get

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