Name: conic sections in polar form
Uploaded: 2025-11-26T05:43:57.000Z
Duration: 1 min 29 s
Channel: JoVE

Une section conique est l’ensemble des points pour lesquels le rapport de la distance à un point fixe, appelé foyer, et une droite fixe, appelée directrice, est constant.Ce rapport est appelé excentricité, et il détermine la forme de la conique : une ellipse si elle est comprise entre zéro et un, une parabole si un, et une hyperbole si elle est supérieure à un.Lorsque le foyer est à l’origine en coordonnées polaires, une équation polaire unique décrit toutes les coniques en utilisant la distance radiale, l’angle polaire, l’excentricité et la distance à la directrice.Si le directeur est vertical, l’équation utilise le cosinus, qui mesure le déplacement horizontal sous forme polaire.Un directeur à droite du foyer donne un signe plus, tandis qu’un autre à gauche donne un signe moins, indiquant sa position le long de l’axe horizontal positif ou négatif.Si le directeur est horizontal, l’équation utilise le sinus, qui mesure le déplacement vertical sous forme polaire.Un directeur au-dessus du focus donne un signe plus, tandis qu’un autre en dessous donne un signe moins, indiquant si le directrix se trouve sur l’axe vertical positif ou négatif.Les équations polaires des coniques aident également à concevoir des dômes en définissant des courbes qui répartissent uniformément les charges structurelles.

Équations polaires des coniques

Une conique peut &#234;tre d&#233;finie en coordonn&#233;es polaires comme l&#8217;ensemble de tous les points dont la distance &#224; un point fixe, appel&#233; foyer, est dans un rapport constant avec leur distance &#224; une droite fixe, appel&#233;e directrice. Ce rapport constant est appel&#233; excentricit&#233;. Cette d&#233;finition unifie tous les types de coniques, ellipses, paraboles et hyperboles, au sein d&#8217;un cadre unique. Lorsque le foyer est situ&#233; &#224; l&#8217;origine du syst&#232;me de coordonn&#233;es polaires, une seule &#233;quation polaire peut d&#233;crire n&#8217;importe quelle conique, ce qui simplifie &#224; la fois leur repr&#233;sentation et leur analyse.L&#8217;&#233;quation polaire g&#233;n&#233;rale d&#8217;une conique est :<figure class='image image-style-align-center image_resized' style='display:table;margin-left:auto;margin-right:auto;width:50%;'><img style='aspect-ratio:1164/173;' src='https://cloudfront.jove.com/files/ftp_upload/40091/40091_Equation_1_editor_40091.svg' alt='40091_Equation_1.svg' /></figure>Ici, la variable r repr&#233;sente la distance entre l&#8217;origine et un point de la conique, &#952; l&#8217;angle polaire, d la distance du foyer &#224; la directrice et e l&#8217;excentricit&#233;. Le choix entre cosinus et sinus d&#233;pend de l&#8217;orientation de la directrice, tandis que le signe indique son orientation par rapport au foyer.La nature pr&#233;cise de la section conique d&#233;pend enti&#232;rement de la valeur de son excentricit&#233;. Lorsque l'excentricit&#233; est &#233;gale &#224; un, la conique est appel&#233;e parabole. Lorsque l'excentricit&#233; est sup&#233;rieure &#224; z&#233;ro mais inf&#233;rieure &#224; un, la conique est une ellipse. Lorsque l'excentricit&#233; est sup&#233;rieure &#224; un, la conique est une hyperbole.Si la directrice est verticale, l'&#233;quation fait intervenir la fonction cosinus. Une directrice situ&#233;e &#224; droite du foyer donne un signe plus avec le cosinus, tandis qu'une directrice situ&#233;e &#224; gauche donne un signe moins. Lorsque la directrice est horizontale, l'&#233;quation fait intervenir la fonction sinus. Une directrice situ&#233;e au-dessus du foyer donne un signe plus avec le sinus, et une directrice situ&#233;e en dessous donne un signe moins. Ces variations permettent de pr&#233;ciser l'orientation et la position de la conique dans le plan.

Une conique peut &#234;tre d&#233;finie en coordonn&#233;es polaires comme l&#8217;ensemble de tous les points dont la distance &#224; un point fixe, ...

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Polar Equations of Conics

conic sections in polar form

Conic sections can be written in polar form using a focus and a directrix. The focus is a fixed point, and the directrix is a fixed line. A point on the conic is defined by the ratio of its distance to the focus and its distance to the directrix. That ratio is called the eccentricity.

This setup gives one framework for ellipses, parabolas, and hyperbolas. When the focus is placed at the origin of the polar coordinate system, one polar equation can represent any conic section. That makes the graph and analysis of conics easier to handle in polar coordinates. In the general equation, r is the distance from the origin to a point on the conic, θ is the polar angle, d is the distance from the focus to the directrix, and e is the eccentricity.

The form of the equation also depends on the direction of the directrix. A vertical directrix uses cosine. If the directrix is to the right of the focus, the cosine form uses a plus sign. If the directrix is to the left, it uses a minus sign. A horizontal directrix uses sine instead.

The sign with sine shows whether the directrix is above or below the focus. A directrix above the focus gives a plus sign with sine. A directrix below the focus gives a minus sign. The value of the eccentricity tells which conic you have. When e = 1, the conic is a parabola. When 0 1, it is a hyperbola.

Conic Sections in Polar Form

inclination of a line

Slope Angle and Line Direction

The slope angle shows how a line tilts on the Cartesian plane. It gives the line's direction by linking geometry with slope, which tells how steeply the line rises or falls. A line has no thickness, but its orientation still matters. This angle is measured counterclockwise from the positive x-axis. For nonhorizontal lines, the inclination angle ranges from 0 to π radians. That range helps describe the line's tilt in a clear geometric way. When two lines intersect, each line has its own...

parabolas

Parabolas: Focus, Directrix, and Symmetry

A parabola is a conic section with a clear geometric rule. It forms when a plane cuts a double-napped cone parallel to the cone’s slant height. The result is an open curve used in coordinate geometry and defined by a fixed point and a fixed line. That fixed point is the focus, and the fixed line is the directrix. Every point on a parabola stays the same distance from the focus as from the directrix. In mathematics, this set of points is called the locus of points with equal distance from both.<br...

reflective property of parabolas

Parabola Focus and Signal Reflection

A parabola is a U-shaped conic section with an important reflective property. It forms when a plane cuts a double-napped cone in a direction parallel to one side of the cone. In geometry, a parabola is also defined as the set of points that are equally far from a fixed point, called the focus, and a fixed line, called the directrix. This curve has a useful way of handling light, sound, and other parallel rays. Incoming rays that travel parallel to the axis of symmetry reflect toward one point.

ellipses

Ellipse Geometry: Foci, Axes, and Equations

An ellipse is a closed curve in geometry with two foci, a major axis, and a minor axis. It is formed when a right circular cone is cut by an inclined plane that does not pass through the cone’s base. The shape is symmetric, which makes its structure easy to study in analytic geometry. One key way to define an ellipse is by distance. For any point on the curve, the sum of the distances to the two fixed points, called the foci, stays constant. That constant sum is equal to the length of the...

eccentricity of an ellipse

Ellipse Shape and Orbits Explained

An ellipse is a key conic section with two foci, or fixed points inside the shape. For any point on the ellipse, the sum of the distances to the two foci stays the same. A pencil, string, and two pins can show this shape in a simple way. If the string is held tight while the pencil moves, it traces an ellipse. The size and shape of an ellipse depend on its eccentricity, written as e. Eccentricity is the ratio of the distance from the center to a focus, c, to the semi-major axis length, a. When...

hyperbolas

Hyperbolas: Foci, Vertices, and Asymptotes

Hyperbolas are conic sections with two separate branches. They form when a plane cuts through a double-napped cone at a steeper angle than the cone’s slope, so the plane passes through both nappes. The two curves are mirror images and open away from each other along the transverse axis. The closest points on the branches to the center are called vertices. The distance from the center to a vertex is labeled a. The axis perpendicular to the transverse axis is the conjugate axis, and it is linked...

geometry of hyperbolas

Hyperbola Properties and Asymptotes

A hyperbola is a curve defined by two fixed points called foci. For any point on the curve, the absolute difference in its distances to the foci stays constant. This distance rule gives the hyperbola its shape and separates it from other conic sections. The standard equation shows two branches that extend forever. Each branch moves closer to two asymptotes, which are lines that guide the curve as it grows. The parameters a and b control key features of the graph. The value a measures the...

polar equations of conics

Conic sections can be written in polar form using a focus and a directrix. The focus is a fixed point, and the directrix is a fixed line. A point on the conic is defined by the ratio of its distance to the focus and its distance to the directrix. That ratio is called the eccentricity. This setup gives one framework for ellipses, parabolas, and hyperbolas. When the focus is placed at the origin of the polar coordinate system, one polar equation can represent any conic section. That makes the...

key terms and concepts

Learn how ellipses, parabolas, and hyperbolas are written in polar form using a focus, directrix, and eccentricity, plus the cosine and sine cases.

Conic Sections in Polar Form | JoVE K12

The JoVE Math Fundamentals collection is designed to build a deep and comprehensive understanding of essential mathematics. Covering the foundations of algebra, geometry, trigonometry, and analytic reasoning, this course establishes a strong base for future studies. The collection connects core mathematical concepts with real-world practice, equipping learners to solve complex problems, master problem-solving skills, and apply abstract ideas.

math fundamentals

Conic Sections in Polar Form

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