Name: parabolas: focus, directrix, and symmetry
Uploaded: 2025-11-26T05:44:12.000Z
Duration: 1 min 30 s
Channel: JoVE

Les sections coniques sont des courbes formées lorsqu’un plan coupe un cône à double nappe. Si le plan est parallèle à l’inclinaison du cône, la courbe résultante est une parabole.Une parabole est un ensemble de points équidistants d’un point fixe – le foyer – et d’une ligne fixe – le directrice.L’axe de symétrie passe par le sommet. Le foyer se trouve le long de cet axe, tandis que le directeur est perpendiculaire à celui-ci du côté opposé.La forme standard d’une parabole découle de sa définition géométrique, où les distances au foyer et au directeur sont égales.L’application de la formule de distance aux deux distances et la mise au carré de chaque côté éliminent la racine carrée.L’extension des deux expressions et la simplification suppriment les termes courants et révèlent la forme standard lorsque l’axe est vertical.L’inversion de x et y donne la forme standard d’un axe horizontal. Le décalage du sommet par rapport à l’origine modifie encore l’équation.La parabole s’ouvre vers le haut ou vers la droite si le foyer se trouve dans la direction positive du sommet. Il s’ouvre vers le bas ou vers la gauche si la mise au point se trouve dans la direction négative.Ces formes et orientations paraboliques apparaissent dans des structures, comme des ponts suspendus ou des antennes paraboliques.

paraboles

Parabolas

Une parabole est une courbe fondamentale de la famille des coniques, issue de l&#8217;intersection d&#8217;un plan avec un c&#244;ne &#224; deux nappes lorsque le plan est parall&#232;le &#224; une g&#233;n&#233;ratrice du c&#244;ne. Cette condition g&#233;om&#233;trique produit une courbe ouverte unique d&#233;finie par le fait que sa distance &#224; un point fixe, le foyer, est &#233;gale &#224; sa distance &#224; une droite fixe, la directrice.Une parabole est math&#233;matiquement d&#233;finie comme le lieu g&#233;om&#233;trique de tous les points d&#8217;un plan &#233;quidistants du foyer et de la directrice. En coordonn&#233;es cart&#233;siennes, la forme standard d&#233;pend de l&#8217;orientation des axes :Pour une parabole orient&#233;e verticalement, la forme standard est&#160;:<figure class='image image-style-align-center image_resized' style='display:table;margin-left:auto;margin-right:auto;width:50%;'><img style='aspect-ratio:902/87;' src='https://cloudfront.jove.com/files/ftp_upload/40087/40087_Equation_1_editor_40087.svg' alt='40087_Equation_1.svg' /></figure>Pour une parabole orient&#233;e horizontalement, la forme standard est&#160;:<figure class='image image-style-align-center image_resized' style='display:table;margin-left:auto;margin-right:auto;width:50%;'><img style='aspect-ratio:902/87;' src='https://cloudfront.jove.com/files/ftp_upload/40087/40087_Equation_2_editor_40087.svg' alt='40087_Equation_2.svg' /></figure>Ici, (h, k) d&#233;signe le sommet de la parabole, et p la distance du sommet au foyer. Si le sommet est &#224; l&#8217;origine, l&#8217;&#233;quation se simplifie en y^2 = 4px et x^2 = 4py, respectivement. Le signe de p d&#233;termine si la parabole s&#8217;ouvre dans le sens positif ou n&#233;gatif le long de l&#8217;axe.L&#8217;axe de sym&#233;trie d&#8217;une parabole est une droite passant par le sommet et le foyer, perpendiculaire &#224; la directrice. La position du foyer par rapport au sommet d&#233;termine la direction d&#8217;ouverture : vers le haut ou vers la droite si le foyer se situe dans le sens positif de l&#8217;axe, et vers le bas ou vers la gauche s&#8217;il se situe dans le sens n&#233;gatif. Le sommet, point &#233;quidistant du foyer et de la directrice, constitue une r&#233;f&#233;rence essentielle pour d&#233;finir la sym&#233;trie et l&#8217;orientation de la courbe.Les paraboles pr&#233;sentent une propri&#233;t&#233; de r&#233;flexion importante&#160;: la tangente en tout point de la parabole forme des angles &#233;gaux avec la droite reliant ce point au foyer et avec l&#8217;axe de sym&#233;trie, ce qui d&#233;coule de la loi de la r&#233;flexion et de la sym&#233;trie g&#233;om&#233;trique de la courbe.

Paraboles

Une parabole est une courbe fondamentale de la famille des coniques, issue de l&#8217;intersection d&#8217;un plan avec un c&#244;ne &#224; deux nappes ...

Education

Enseignement

JoVE Core

Math Fundamentals

Fondamentaux des mathématiques

Unit

Unité

Analytic Geometry

Géométrie analytique

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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.

A parabola can open in different directions based on its orientation. For a vertical parabola, the standard form uses y and has a vertex at (h, k). For a horizontal parabola, the standard form uses x and also places the vertex at (h, k). When the vertex is at the origin, the equations simplify to y2 = 4px and x2 = 4py.

The value of p is the distance from the vertex to the focus. Its sign shows whether the parabola opens in the positive or negative direction along its axis. That means it may open upward or downward, or rightward or leftward, depending on where the focus lies.

The vertex is the turning point of the curve and a key reference for its shape. The axis of symmetry passes through the vertex and the focus, and it is perpendicular to the directrix. This central line divides the parabola into two matching halves.

Parabolas also have a reflective property. A tangent line at any point on the curve makes equal angles with the line to the focus and with the axis. This happens because of the law of reflection and the parabola’s geometric symmetry.

Parabolas: Focus, Directrix, and Symmetry

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

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 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...

key terms and concepts

Learn how parabolas are formed, how the focus and directrix define the curve, and how vertex, axis of symmetry, and p affect its direction.

Parabolas: Focus, Directrix, and Symmetry | 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

Parabolas: Focus, Directrix, and Symmetry

Description

Transcript

Tags