Pour les articles homonymes, voir foyer.
On désigne généralement par foyer un ou plusieurs points caractéristiques associés à une figure remarquable de géométrie.
La définition monofocale d'une conique utilise conjointement un foyer F et une droite D appelée directrice associée. La conique apparaît comme ensemble des points M du plan tels que d ( M , F ) = e d ( M , D ) {\displaystyle \ {d(M,F)}=e\ {d(M,D)}} . Selon la valeur du réel strictement positif e qu'on nomme excentricité, l'ensemble sera une ellipse, une parabole ou une hyperbole.
Si la parabole ne possède qu'un foyer, l'ellipse et l'hyperbole en possèdent chacune deux, permettant une définition bifocale de ces courbes. Dans le cas de l'ellipse, la somme des distances du points M aux deux foyers est une constante ; dans le cas de l'hyperbole, c'est la valeur absolue de la différence.
D'autres courbes planes se voient également attribuer des foyers, notamment si leurs points possèdent des propriétés liées aux distances à ces foyers. On peut citer le cas de la lemniscate de Bernoulli, des ovales de Cassini, de certaines cubiques comme la strophoïde...
En este artículo sobre geografía se detectó el siguiente problema. Por favor, edítalo y/o discute el problema en la discusión para mejorarlo: Necesita referencias adicionales para su verificación. Este aviso fue puesto el 3 de septiembre de 2013. Frontera entre Uruguay y Brasil Localización de Uruguay (naranja) y Brasil (verde). Mapa detallado d ela frontera. Brasil Brasil Uruguay Uruguay Longitud total 1068 kmHistoriaCreación 1828Trazado actual 1908[editar datos en Wikida…
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